DOC: `get_data` docstring rewrite

Rewrite of docstring for get_data to give more details on the cache.
DOC: rewrite with doctests

Make get_data examples into doctests.
Merge pull request #287 from matthew-brett/in-memory-doc-rewrite

DOC: `get_data` docstring rewrite

Rewrite of docstring for get_data to give more details on the cache.