Fix docs warning builds

[dgarcia360]

Building the docs raises up to 42 warnings related to xrefs duplications.

I plan to submit a PR fixing them.

[dgarcia360]

#41 should close this issue. There are some additional warnings not strictly linked with docs but with the python-driver implementation.The generator still raises the next warnings:

1. python-driver/cassandra/metadata.py:docstring of cassandra.metadata.group_keys_by_replica:11: WARNING: Block quote ends without a blank line; unexpected unindent.

The function is outside a class.

2. home/dgarcia/python-driver/cassandra/datastax/graph/fluent/_predicates.py:docstring of cassandra.datastax.graph.fluent.predicates.Geo.inside:4: WARNING: Unexpected indentation.

The function might be missing a docstring.

3. python-driver/cassandra/cluster.py:docstring of cassandra.cluster.GraphExecutionProfile.graph_options:1: WARNING: more than one target found for cross-reference 'GraphOptions': cassandra.datastax.graph.GraphOptions, cassandra.graph.GraphOptions

Python doesn't know which class should load. It should be solved by providing the full path.