Fix some usage messages of 'nova volume-*'

'nova volume-show', 'nova volume-delete', 'nova volume-snapshot-show'
and 'nova volume-snapshot-delete' can be specified by ID or name as
the argument.
But that is not described on the usage messages.
This patch fixes the usage messages.

Fixes bug 1093172

Change-Id: I3dcdd2aca923e9304efd9ea7b467d5b81cd642a7
Update README.rst

Update the README to remove PENDING links to non-existent docs, and
to remove Rackspace-specific references.

Change-Id: I08311e0da3d181c3817573be93f6eed6e15e3812
Merge "Update README.rst"