Improve server dump create helptext

The `server dump create` command instructs Nova to trigger a crash
dump in the guest OS. Assuming the guest supports this, the resulting
dump file will be located in the guest, in a location dependent on the
guest OS. Explain all that in the helptext.

Story: 2010384

Change-Id: If940ed5cce6c5ab4193ab1494738149370da9aad

Author: notartom

openstackclient/compute/v2/server.py

@@ -1756,8 +1756,9 @@ class CreateServerDump(command.Command):
 
     Trigger crash dump in server(s) with features like kdump in Linux.
     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.
+    memory, and also crash the server(s). This is contingent on guest operating
+    system support, and the location of the dump file inside the guest will
+    depend on the exact guest operating system.
 
     This command requires ``--os-compute-api-version`` 2.17 or greater.
     """