Document that server dump create requires 2.17

There is no indication to the user in the command
help that they have to use 2.17 or greater [1] to run
the "openstack server dump create" command. This
mentions that requirement in the help of the command.

[1] https://developer.openstack.org/api-ref/compute/#trigger-crash-dump-in-server

Change-Id: I02c06e10a26eb38ddecb70f970cfcbfad962201c

Author: mriedem

openstackclient/compute/v2/server.py

@@ -947,6 +947,8 @@ class CreateServerDump(command.Command):
     It will create a dump file in the server(s) dumping the server(s)'
     memory, and also crash the server(s). OSC sees the dump file
     (server dump) as a kind of resource.
+
+    This command requires ``--os-compute-api-version`` 2.17 or greater.
     """
 
     def get_parser(self, prog_name):