releases/15.0R/relnotes: User-facing description for mountd(8) change

Explain what this change means to users. The description from the
commit message was fairly incomprehensible from this point of view.

Fix the commit hash (it pointed to some MFC commit to stable/14).

While here, fix punctuation around the commit links and "Sponsored by
The FreeBSD Foundation" lines (see also note in the previous commit).