Improve help text of volume create command

This patch adds information about whichever default type is set for
API will be used during volume creation if we don't provide a
volume type in volume create command.
This patch also improves the help text of `cinder type-default` command.

Change-Id: I0c437b2c4f02c12d17c04719cbeff8521647ae15

cinderclient/v3/shell.py

@@ -649,7 +649,10 @@ def do_reset_state(cs, args):
 @utils.arg('--volume-type',
            metavar='<volume-type>',
            default=None,
-           help='Volume type. Default=None.')
+           help='Volume type. Default=None, that is, use the default '
+                'volume type configured for the Block Storage API.  You '
+                "can see what type this is by using the 'cinder type-default'"
+                ' command.')
 @utils.arg('--volume_type',
            help=argparse.SUPPRESS)
 @utils.arg('--availability-zone',

cinderclient/v3/shell_base.py

@@ -693,7 +693,13 @@ def do_type_list(cs, args):
 
 
 def do_type_default(cs, args):
-    """List the default volume type."""
+    """List the default volume type.
+
+    The Block Storage service allows configuration of a default
+    type for each project, as well as the system default, so use
+    this command to determine what your effective default volume
+    type is.
+    """
     vtype = cs.volume_types.default()
     shell_utils.print_volume_type_list([vtype])