Add missing info in Submodule.remove docstring

[EliahKagan]

Fixes #1712

This rewords and adds some missing information to the docstring of Submodule.remove, for the method parameter, discussed in #1712. It uses the second suggestion presented in that issue, formatted in the style of the surrounding docstring (an 88-column wrap).

It also does some other rewording in that docstring, for clarity, at the end of that same section (on the method parameter), and a small punctuation fix in the section about the force parameter.

A note on coordination

Please note that, while this is not a behavioral change, I believe an ordinary patch release rather than a "post" release should be made if this is included. In particular, although I discussed doing a "post" release in #1715, a release that includes both this change and the changes from #1715 should be a normal 3.1.39 release. (In contrast, if this modified only files such as README.md or documentation in doc/, then a post release would make sense.)

[Byron]

Thanks so much for your help, it's much appreciated!

I will happily not make the next release a "post" release as suggested (after looking it up), merely because I have never done it before and am afraid that this breaks anything or anyone downstream 😅.

[EliahKagan]

That makes sense, and as I've just written about in #1716, I think there's another change that might make sense to include in 3.1.39 [edit: or 3.1.40], if the timing works out.

Otherwise, it, or some alternative to it, could come in with 3.1.40 [edit: or 3.1.41] later.