Documentation: CG_MAGIC isn't a magic number

Documentation: CG_MAGIC isn't a magic number

At least not in the sense described here: it delineates UFS cylinder
groups, is never assigned, and the only macro that incorporates it
(ufs_cg_chkmagic; the second one is unused) is used to detect CGs and
protect from filesystem corruption

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Link: https://lore.kernel.org/r/a508477cfeb18eca4a24c29836f809fe34f20467.1663280877.git.nabijaczleweli@nabijaczleweli.xyz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>