Reflect multiattach deprecation in help text

Change Icbb9c0ca89b25620cedff6cac7a4723e7126eca6 notified that the
``multiattach`` argument was deprecated, but nothing was added to the
args help text to indicate this, relying on a user to have read the
release notes.

In preparation of removing this option, this updates the help text so
there is at least some indication that it is going away.

Change-Id: I9e767a3f1411fbfc0bf0e433b45560e451d547d5
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

cinderclient/v2/shell.py

@@ -298,7 +298,7 @@ class CheckSizeArgForCreate(argparse.Action):
 @utils.arg('--allow-multiattach',
            dest='multiattach',
            action="store_true",
-           help=('Allow volume to be attached more than once.'
+           help=('Allow volume to be attached more than once. (DEPRECATED)'
                  ' Default=False'),
            default=False)
 def do_create(cs, args):

cinderclient/v3/shell.py

@@ -568,7 +568,7 @@ def do_reset_state(cs, args):
 @utils.arg('--allow-multiattach',
            dest='multiattach',
            action="store_true",
-           help=('Allow volume to be attached more than once.'
+           help=('Allow volume to be attached more than once. (DEPRECATED)'
                  ' Default=False'),
            default=False)
 @utils.arg('--poll',