DOC: clean-up docstrings of sql + fix doc build
@mangecoeur I don't think I changed some real content, just some adjustments for the numpy docstring standards. Only for has_table I corrected the parameters.

And the second commit should fix the doc builds (see http://pandas-docs.github.io/pandas-docs-travis/io.html#io-sql)

nothing disruptive, so going to merge.
