cephfs: pool safety & print deprecation warning

Print a message indicating to users that this tool
is now deprecated in favour of vxattrs.

Signed-off-by: John Spray <john.spray@redhat.com>
doc: add cephfs layout documentation

This clarifies how to deal with layouts in CephFS
using vxattrs.  We can point people here if they
ask what they should use instead of the deprecated
`cephfs set_layout`.

Signed-off-by: John Spray <john.spray@redhat.com>