filesystems.5, erofs.5: add erofs documentation

filesystems.5, erofs.5: add erofs documentation

Modelled after tmpfs(5) ‒ there's a listing of mount options, and a
summary of limitations.  The feature flags are described in mkfs.erofs,
and they're versioned and maintained upstream quite well there, so no
need to duplicate those, since you only care on image creation.

The real value add is the mount options, but I cannot figure out how
device_id and fsid interact with the system at large, so I just noted
they're there.

State as of Linux 6.3-rc5.

Also, remove explicit .TP indent in filesystems.5 since we're already
touching this hunk: all entries sans iso9660 and Reiserfs fall within
the default prevailing indent, so no need to specify a wide one.

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Alejandro Colomar <alx@kernel.org>