x86/mm: Add a few comments

It's a shame to hide useful comments in Changelogs, add some to the
code.

Shamelessly stolen from commit:

  c40a56a7 ("x86/mm/init: Remove freed kernel image areas from alias mapping")

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221110125544.460677011%40infradead.org