x86: Update comment about pgd_list

x86: Update comment about pgd_list

This venerable comment got detached from its context when the code moved
in commit 394158559d4c ("x86: move all the pgd_list handling to one
place"). Put it back next to its context. It was originally on
pgd_list_add() but it actually describes pgd_list so put it there.

While moving it, update it to strip away stale and superfluous info.
pageattr.c doesn't exist any more. pgd_list is now required
for all x86 architectures. Also be slightly more precise about what PGDs
are in this list.

[ dhansen: tweak and trim the updated comment a bit ]

Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20260323-pgd_list-comment-v2-1-77ccf2dc77e8@google.com