shm: Document shm_create_largepage()

While here, move notes about FreeBSD-specific functionality to the
COMPATIBILITY section, and document the ECAPMODE error for shm_open().

Reviewed by: pauamma, kib
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38282