Document that 'index' may also be a list of strings for most methods (elastic#921)

I discovered whilst researching scoutapp/scout_apm_python#150 that the index parameter for all methods also supports a list of strings, but this isn't documented here. For example `elasticsearch-dsl-py` [always passes a list of strings](https://github.com/elastic/elasticsearch-dsl-py/blob/master/elasticsearch_dsl/search.py#L100). It's [`_make_path`](https://github.com/elastic/elasticsearch-py/blob/master/elasticsearch/client/utils.py#L44) that allows this to happen. This updates the relevant methods' documentation to indicate that it is supported.

Author: adamchainz

elasticsearch/client/__init__.py

@@ -710,7 +710,8 @@ def search(self, index=None, body=None, params=None):
  Execute a search query and get back search hits that match the query.
  `<http://www.elastic.co/guide/en/elasticsearch/reference/current/search-search.html>`_
 
- :arg index: A comma-separated list of index names to search; use `_all`
+ :arg index: A list of index names to search, or a string containing a
+ comma-separated list of index names to search; use `_all`
  or empty string to perform the operation on all indices
  :arg body: The search definition using the Query DSL
  :arg _source: True or false to return the _source field or not, or a
@@ -853,8 +854,9 @@ def update_by_query(self, index, body=None, params=None):
  Perform an update on all documents matching a query.
  `<https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-update-by-query.html>`_
 
- :arg index: A comma-separated list of index names to search; use `_all`
- or empty string to perform the operation on all indices
+ :arg index: A list of index names to search, or a string containing a
+ comma-separated list of index names to search; use `_all` or the
+ empty string to perform the operation on all indices
  :arg body: The search definition using the Query DSL
  :arg _source: True or false to return the _source field or not, or a
  list of fields to return
@@ -1024,8 +1026,9 @@ def delete_by_query(self, index, body, params=None):
  Delete all documents matching a query.
  `<https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-delete-by-query.html>`_
 
- :arg index: A comma-separated list of index names to search; use `_all`
- or empty string to perform the operation on all indices
+ :arg index: A list of index names to search, or a string containing a
+ comma-separated list of index names to search; use `_all` or the
+ empty string to perform the operation on all indices
  :arg body: The search definition using the Query DSL
  :arg _source: True or false to return the _source field or not, or a
  list of fields to return
@@ -1115,8 +1118,9 @@ def search_shards(self, index=None, params=None):
  out issues or planning optimizations with routing and shard preferences.
  `<http://www.elastic.co/guide/en/elasticsearch/reference/current/search-shards.html>`_
 
- :arg index: A comma-separated list of index names to search; use `_all`
- or empty string to perform the operation on all indices
+ :arg index: A list of index names to search, or a string containing a
+ comma-separated list of index names to search; use `_all` or the
+ empty string to perform the operation on all indices
  :arg allow_no_indices: Whether to ignore if a wildcard indices
  expression resolves into no concrete indices. (This includes `_all`
  string or when no indices have been specified)
@@ -1153,8 +1157,9 @@ def search_template(self, index=None, body=None, params=None):
  fill in template parameters.
  `<http://www.elastic.co/guide/en/elasticsearch/reference/current/search-template.html>`_
 
- :arg index: A comma-separated list of index names to search; use `_all`
- or empty string to perform the operation on all indices
+ :arg index: A list of index names to search, or a string containing a
+ comma-separated list of index names to search; use `_all` or the
+ empty string to perform the operation on all indices
  :arg body: The search definition template and its params
  :arg allow_no_indices: Whether to ignore if a wildcard indices
  expression resolves into no concrete indices. (This includes `_all`
@@ -1347,7 +1352,8 @@ def count(self, doc_type=None, index=None, body=None, params=None):
  Execute a query and get the number of matches for that query.
  `<http://www.elastic.co/guide/en/elasticsearch/reference/current/search-count.html>`_
 
- :arg index: A comma-separated list of indices to restrict the results
+ :arg index: A list of index names or a string containing a
+ comma-separated list of index names to restrict the results to
  :arg body: A query to restrict the results specified with the Query DSL
  (optional)
  :arg allow_no_indices: Whether to ignore if a wildcard indices
@@ -1453,7 +1459,8 @@ def msearch(self, body, index=None, params=None):
 
  :arg body: The request definitions (metadata-search request definition
  pairs), separated by newlines
- :arg index: A comma-separated list of index names to use as default
+ :arg index: A list of index names, or a string containing a
+ comma-separated list of index names, to use as the default
  :arg max_concurrent_searches: Controls the maximum number of concurrent
  searches the multi search api will execute
  :arg pre_filter_shard_size: A threshold that enforces a pre-filter
@@ -1672,7 +1679,8 @@ def msearch_template(self, body, index=None, params=None):
 
  :arg body: The request definitions (metadata-search request definition
  pairs), separated by newlines
- :arg index: A comma-separated list of index names to use as default
+ :arg index: A list of index names, or a string containing a
+ comma-separated list of index names, to use as the default
  :arg max_concurrent_searches: Controls the maximum number of concurrent
  searches the multi search api will execute
  :arg search_type: Search operation type, valid choices are:
@@ -1699,8 +1707,9 @@ def field_caps(self, index=None, body=None, params=None):
  The field capabilities API allows to retrieve the capabilities of fields among multiple indices.
  `<http://www.elastic.co/guide/en/elasticsearch/reference/current/search-field-caps.html>`_
 
- :arg index: A comma-separated list of index names; use `_all` or empty
- string to perform the operation on all indices
+ :arg index: A list of index names, or a string containing a
+ comma-separated list of index names; use `_all` or the empty string
+ to perform the operation on all indices
  :arg body: Field json objects containing an array of field names
  :arg allow_no_indices: Whether to ignore if a wildcard indices
  expression resolves into no concrete indices. (This includes `_all`