DOC: make links actually link to API docs
There are some places in the documentation where a link to the API docs is provided, e.g., `:func:`~pandas.read_html``, but no link is made. This is because the fully resolved name (i.e., the module where the function is defined [no package references]) must be used, i.e., `:func:`~pandas.io.html.read_html``. This particular example is fixed, but @jreback mentioned that he's seen them in other places.
