DOC: make links actually link to API docs

[cpcloud]

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.

[jreback]

just a couple I guess

../io.rst:
   36 : :func:`~pandas.io.parsers.read_csv` and :func:`~pandas.io.parsers.read_table`.
  333 : :func:`~pandas.io.parsers.read_csv` and :func:`~pandas.io.parsers.read_table`
  657 : While `read_csv` reads delimited data, the :func:`~pandas.io.parsers.read_fwf`
  928 : The toplevel :func:`~pandas.io.parsers.read_html` function can accept an HTML
 1837 : connection, just use the :func:`~pandas.io.sql.read_frame` function to get the
 1886 : The function :func:'~pandas.io.StataWriter.write_file' will write a DataFrame 
 1905 : initialization. Its function :func:'~pandas.io.StataReader.data' will 
 1921 : Alternatively, the function :func:'~pandas.io.read_stata' can be used
./missing_data.rst:
   71 : pandas provides the :func:`~pandas.core.common.isnull` and
   72 : :func:`~pandas.core.common.notnull` functions, which are also methods on

[cpcloud]

toplevel isnull and notnull are not linking because there's no API docs generated for them. same for read stata. read_csv and read_table are working

• read_csv
• read_table
• read_html
• read_fwf
• read_clipboard
• stata stuff
• put pandas.io.stata and pandas.io.sql in pandas.io.__init__
• correctly link pandas.io.sql
• fix up isnull, notnull docstrings