Add missing '(name only)' message for keypair in computev2

In 'keypair show' command. One help message had '(name only)'
but the doc had not. And another had not '(name only)' but the
doc had. This patch Fixs them and keeps the consistent between
doc and help message.

Change-Id: Ic88219ae25a6f545a1db39e6b585e1857ae9e2e0

Author: Huanxuan Ao

doc/source/command-objects/keypair.rst

@@ -70,4 +70,4 @@ Display public key details
 
 .. describe:: <key>
 
-    Public key to display
+    Public key to display (name only)

openstackclient/compute/v2/keypair.py

@@ -146,7 +146,7 @@ def get_parser(self, prog_name):
             '--public-key',
             action='store_true',
             default=False,
-            help=_("Show only bare public key")
+            help=_("Show only bare public key (name only)")
         )
         return parser