Update documentation for text format

In commit 000ca91a the text format was extended to also include
information about the master node. This commit updates the
htools man page to reflect this change.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>

man/htools.rst

@@ -173,7 +173,8 @@ support all options. Some common options are:
   - node total disk
   - node free disk
   - node physical cores
-  - offline field (as ``Y`` or ``N``)
+  - offline/role field (``Y`` for offline nodes, ``N`` for online non-master
+    nodes, and ``M`` for the master node which is always online)
   - group UUID
   - node spindle count