releases/15.0R/relnotes: Fix and expand paragraphs on ps(1)

Improve description of what was changed.

Add missing description of the change on matching user's processes using
the effective user ID under "Userland", and of the manual page revamp
under "Man Pages".

Remove paragraphs under "Man Pages" that in fact refer to commits to
RELNOTES. These are not manual page changes, and instead the *content*
of the commit should appear in the release notes. The relevant
information has been added under "Userland".

Re-order changes chronologically.

Fix commit hashes (they pointed to MFC commits to stable/14).

While here, in the change portions, fix punctuation around the commit
links and "Sponsored by The FreeBSD Foundation" lines to match what
I think was common practice in earlier release notes.

Sponsored by: The FreeBSD Foundation