Add inline documentation for atomlib.php

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Doc: Add $msg parameter entry for _p($msg) function

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[SergeyBiryukov]

Hi there, welcome to WordPress Trac! Thanks for the ticket.

Similarly to class-pclzip.php in #49163, atomlib.php used to be treated as an external library and was previously excluded from any documentation or coding standard fixes. We only modify third-party libraries when it's needed to fix a bug, like in [37461], [38883], or [39591].

That said, looking at the commit history, there were some string changes in the past, so I think it's safe to treat atomlib.php as "adopted" library rather than external. Related: #50498.

If we were to improve the documentation though, I think it would be better to document all the functions in that file, rather than one at a time.

Trac ticket: core.trac.wordpress.org/ticket/51306

This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See ​GitHub Pull Requests for Code Review in the Core Handbook for more details.