Placeholder elaboration in function-modifiers.rst

[The-Arbiter]

Hi,

This PR is a continuation of Create placeholders.rst #13239

Thanks @cameel for the feedback, I have removed the mutex code example and cleaned things up a bit as well as implemented all of the changes in your review.

Looking back on it, I agree with you that placeholders didn't really deserve their own page in the docs. This version just adds two paragraphs to function-modifiers prior to the first mention of the _ character in the section. This was the area where I feel it best fits within this page.

Please let me know if you have any other recommendations or changes.

Thanks, Arby

[The-Arbiter]

Shortened changes to one paragraph.

[leonardoalt]

I fixed the merge commit and rebased it.