bug#52202: 29.0.50; try-completion, all-completions support for a list of strings COLLECTION is not documented in their docstrings

[Matt Armstrong]

`try-completion' and `all-completions' support a completion COLLECTION
of a list of strings.  This is not documented in their docstrings.  This
is confusing because every other supported data structure is mentioned.

This support was added around 20 years ago.  See Stefan's commit
695deb1857 or
https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=695deb1857dd95239df0139894e40719699a6cc8

I'm assuming this was just an oversight, since `test-completion'
mentions "list of strings" as does the description of `try-completion'
in the elisp manual.