Fix "server unset" document issue

Fix "--property" option describe issue in "server unset"
document, and update the help message to keep consistent.

Change-Id: I68022a187e83fad6320365400ad2a1b0c8cf9a61

Author: kiwik

doc/source/command-objects/server.rst

@@ -764,7 +764,7 @@ Unset server properties
 
 .. option:: --property <key>
 
-    Property key to remove from server (repeat to set multiple values)
+    Property key to remove from server (repeat to remove multiple values)
 
 .. describe:: <server>
 

openstackclient/compute/v2/server.py

@@ -1738,7 +1738,7 @@ def get_parser(self, prog_name):
             action='append',
             default=[],
             help=_('Property key to remove from server '
-                   '(repeat to unset multiple values)'),
+                   '(repeat to remove multiple values)'),
         )
         return parser