Document the VFS FS events

MOUNT notifies when a filesystem is mounted
REMOUNT notifies when a filesystem is mounted again
UNMOUNT notifies when a filesystem is unmounted

These events are asynchronous to the actual state of the event (though the data
is recorded at a time when it is stable). The mount event is reported after the
filesystem is mounted. However, in the interim it may be unmounted by another
agent. Likewise, umount is called just before the mountpoint is finished tearing
down. It may be remounted (or maybe if the process scheduling is wonky and devd
gets to run before the last few steps are complete).

Sponsored by: Netflix
Diffential Revision: https://reviews.freebsd.org/D25969