The following is a screenshot of QBasic 1.1 in MS DOS 6.22
That would make time frame for your question 1994 or later, right?
The meaning of the ellipsis is: This is not an immediate action, there will be another input required from the user, and thus a new dialog opens.
Erm, yes, but. Ellipsis stand for continuation. Like when printed on the end of a page to mark continuation on the next For a menu this means that its command isn't executed right away but needs additional input, thus followed by some dialog or other means of further specification.
Was this meaning just a common sense, or did Microsoft (or someone else) have written guidelines for this usability rule?
No need for such as it was standing UI practice by that time - beside being a typographic convention for'continuation' way before any computer use. Not just for Microsoft, which used it at least a decade before, but across the industry since the 1970s.
Of course does Microsoft Windows 3.1/3.11 of that time (1994) use ellipses in menus in a linage going back 9 years prior, 1985, to version 1.x:

(Taken from Wikipedia)
It might be important to keep in mind that this is not a Windows only thing, as DOS applications did show ellipsis for continuation as well early on, not just with Microsoft, but all across the industry. Take Turbo Pascal 6.0 of 1990 as an example:

(Taken from Wikipedia)
This example - looking quite like above QuickBasic screen - also shows the strong influence IBM's 1987 CUA had on the PC software, as both are based thereon. Borland further supporting proliferation by makingTurbo Vision available.
Continuing with GUI, before Windows Apples Mac (*1) used it with the very first System 1 of 1984:

(Image taken from this short video)
Note that the Mac even included a dedicated ellipsis character (X'C9') for this. Of course it 'just' took it from the Lisa before:

(Taken from this Low End Mac article about the Lisa)
Apples development is well known as low effort copy of Xerox' smaltalk based systems, where ellipses can be found all the way back to he very first implementations - like visible on this Smalltalk 74 screen on a Alto (*2):

(Tanken from this nice Smalltalk overview at the CHM website)
... this could as well be continued with text UI examples back into mainframe time marking continuation, but I'd think continued usage is already pretty clear.
*1 - Which, without any doubt, had some influence on Windows.
*2 - Not the best example, pictures are hard to find. Not just because of age,but also because Smalltalk isn't as menu heavy as today. No, not booting up any Star right now :)