15.0R/relnotes: Add notes for commits mentioning relnotes, batch 5

Add more content coming from RELNOTES.

RELNOTES processing is now finished, except for the last entry about
deprecated platforms which in its current form is outdated and must be
reworked.

Since my mail to developers@, didn't make direct progress on the result of:
git log --first-parent -E --grep='[rR][eE][lL]([eE][aA][sS][eE]:space:+)?[nN][oO][tT][eE]' origin/releng/14.3..origin/releng/15.0
so was at about at ~25 commits from the start (in reverse chronological order).
But now, all reamining RELNOTES commits can be skipped as they have been handled by copying and adapting the content of the latest RELNOTES.

Sponsored by: The FreeBSD Foundation