The layout of the `--edit-key` listing is not documented (not that I could find anyway). The abbreviations you mention however are, somewhere in the info pages (`info gpg`).

I searched for `S:` and found that I actually wanted to search for `usage:`.

In "GPG Key related Options":

> 4.2.1 How to change the configuration
> -------------------------------------
> 
> These options are used to change the configuration and are usually
> found in the option file. 
>
> '--list-options parameters'
>
> show-usage
>
> Show usage information for keys and subkeys in the standard
> key listing. This is a list of letters indicating the allowed
> usage for a key ('E'=encryption, 'S'=signing,
> 'C'=certification, 'A'=authentication). Defaults to no.

So, doing `gpg -k --list-options show-usage 1A3ABKEY` will show you something like this:

 pub rsa4096/1A3ABKEY 2015-01-25 [SC]
 uid [ultimate] Some Key
 sub rsa4096/4B907KEY 2015-09-19 [S]
 sub rsa4096/F9A41KET 2015-09-19 [E]


Some more info is found in "Unattended Usage of GPG"

> Key-Usage: USAGE-LIST
>
> Space or comma delimited list of key usages. Allowed values are
> 'encrypt', 'sign', and 'auth'. This is used to generate the key
> flags. Please make sure that the algorithm is capable of this
> usage. Note that OpenPGP requires that all primary keys are
> capable of certification, so no matter what usage is given here,
> the 'cert' flag will be on. If no 'Key-Usage' is specified and the
> 'Key-Type' is not 'default', all allowed usages for that particular
> algorithm are used; if it is not given but 'default' is used the
> usage will be 'sign'.

So, while not immediately apparent, the info is there, somewhere, on your system. If `man` does not help you, try `man -k` and/or `info`.