Doc: based: Add some COMPAT flags to legacy code

These are blocks that don't involve cib_legacy_mode() or the legacy_mode
variable explicitly. The COMPAT flags make them slightly easier to find
when we're cleaning up at a compatibility break.

F_CIB_GLOBAL_UPDATE can be set either by legacy code (the broadcast
block in send_peer_reply) or by sync_our_cib(). The sync_our_cib() case
might remain useful outside of a legacy context, I'm not entirely sure.
I've only marked the uses that are definitely legacy.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>