malloc(9): Document/complete aligned variants

Comments on a pending kvmclock driver suggested adding a
malloc_aligned() to complement malloc_domainset_aligned(); add it now,
and document both.

(cherry picked from commit 6162cf885c00a0893a0961415f1829942343dcc1)
(cherry picked from commit 04cc0c393c317b6d5e28c8dc80cd1b5ea071f28f)