fscrypt: document that CephFS supports fscrypt now

The help text for CONFIG_FS_ENCRYPTION and the fscrypt.rst documentation
file both list the filesystems that support fscrypt.  CephFS added
support for fscrypt in v6.6, so add CephFS to the list.

Link: https://lore.kernel.org/r/20231227045158.87276-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>