doc: disable cephx requires auth_supported = none

doc: disable cephx requires auth_supported = none

    auth cluster required = none
    auth service required = none
    auth client required = none

is not enough to disable cephx if there is a

    auth_supported = cephx

line in the [global] section. It will produce the following error

    # ceph --id myself auth list
    2013-10-13 11:05:00.368638 7f812aea3700  0 librados: client.myself authentication error (1) Operation not permitted
    Error connecting to cluster: PermissionError

Changing the line to

    auth_supported = none

fixes the problem.

Signed-off-by: Loic Dachary <loic@dachary.org>