Trivial: Improve doc for "server create" command

The following options of "server create" command support searching
by both name and ID. So add this info into doc.

--image, --volume, --flavor, --security.

Change-Id: I93b167da58144e5de6c9996009b7ea2449fb4cd8

Author: tang-chen

doc/source/command-objects/server.rst

@@ -77,19 +77,20 @@ Create a new server
 
 .. option:: --image <image>
 
-    Create server from this image
+    Create server from this image (name or ID)
 
 .. option:: --volume <volume>
 
-    Create server from this volume
+    Create server from this volume (name or ID)
 
 .. option:: --flavor <flavor>
 
-    Create server with this flavor
+    Create server with this flavor (name or ID)
 
 .. option:: --security-group <security-group-name>
 
-    Security group to assign to this server (repeat for multiple groups)
+    Security group to assign to this server (name or ID)
+    (repeat for multiple groups)
 
 .. option:: --key-name <key-name>
 

openstackclient/compute/v2/server.py

@@ -276,25 +276,25 @@ def get_parser(self, prog_name):
         disk_group.add_argument(
             '--image',
             metavar='<image>',
-            help=_('Create server from this image'),
+            help=_('Create server from this image (name or ID)'),
         )
         disk_group.add_argument(
             '--volume',
             metavar='<volume>',
-            help=_('Create server from this volume'),
+            help=_('Create server from this volume (name or ID)'),
         )
         parser.add_argument(
             '--flavor',
             metavar='<flavor>',
             required=True,
-            help=_('Create server with this flavor'),
+            help=_('Create server with this flavor (name or ID)'),
         )
         parser.add_argument(
             '--security-group',
             metavar='<security-group-name>',
             action='append',
             default=[],
-            help=_('Security group to assign to this server '
+            help=_('Security group to assign to this server (name or ID) '
                    '(repeat for multiple groups)'),
         )
         parser.add_argument(