Extend some docstrings with useful information

This patch extends some of the docstrings in the `Client` class with
more useful information. The patch also adds `intersphinx` to allow for
linking to other docs within zaqarclient. For example:

    :rtype: :class:`zaqarclient.transport.base.Transport`

will link to the documentation for Transport if it has been generated.

Change-Id: I928d63d74739e9e18709979e79c95b5606d97032
Merge "Extend some docstrings with useful information"