Fix explanation of gnt-node evacuate --primaries-only

Furthermore, correct the --help display on evacuate.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

lib/client/gnt_node.py

 #
 #
 
-# Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Google Inc.
+# Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Google Inc.
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -865,8 +865,7 @@ commands = {
     [FORCE_OPT, IALLOCATOR_OPT, NEW_SECONDARY_OPT, EARLY_RELEASE_OPT,
      PRIORITY_OPT, PRIMARY_ONLY_OPT, SECONDARY_ONLY_OPT],
     "[-f] {-I <iallocator> | -n <dst>} <node>",
-    "Relocate the secondary instances from a node"
-    " to other nodes"),
+    "Relocate the primary and/or secondary instances from a node"),
   "failover": (
     FailoverNode, ARGS_ONE_NODE, [FORCE_OPT, IGNORE_CONSIST_OPT,
                                   IALLOCATOR_OPT, PRIORITY_OPT],

man/gnt-node.rst

@@ -118,7 +118,12 @@ potential recovery).
 Note that this command is equivalent to using per-instance commands for
 each affected instance individually:
 
-- ``--primary-only`` is equivalent to ``gnt-instance failover/migration``
+- ``--primary-only`` is equivalent to ``gnt-instance
+  failover/migration`` for non-DRBD instances, but for DRBD instances
+  it's different, and usually is a slow process (it will change the
+  primary to another node while keeping the secondary, this requiring
+  data copies, whereas failover/migrate will only toggle the
+  primary/secondary roles, a fast process)
 - ``--secondary-only`` is equivalent to ``gnt-instance replace-disks``
   in the secondary node change mode (only valid for DRBD instances)
 - when neither of the above is done a combination of the two cases is run