rbd: mention namespace in "rbd mirror pool" command descriptions

rbd: mention namespace in "rbd mirror pool" command descriptions

Commit 5e64748927d0 ("doc/rbd: add namespace information for mirror
commands") did this for the man page, update the built-in help as well.

The "by default" bit in the description of "rbd mirror pool enable" and
"rbd mirror pool disable" commands is specific to pool mode which is in
turn specific to journal-based mirroring, so it's removed.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 7e02946a3f60da0dcc99e0b4a4b6b3d359db1c15)