Astropy: Make docstring of fits.open more compliant with numpydoc

[astrofrog]

There is a section in the docstring of fits.open that looks like:

kwargs : dict, optional
    additional optional keyword arguments, possible values are:

    - **uint** : bool

        Interpret signed integer data where ``BZERO`` is the
        central value and ``BSCALE == 1`` as unsigned integer
        data.  For example, ``int16`` data with ``BZERO = 32768``
        and ``BSCALE = 1`` would be treated as ``uint16`` data.
        This is enabled by default so that the pseudo-unsigned
        integer convention is assumed.

        Note, for backward compatibility, the kwarg **uint16** may
        be used instead.  The kwarg was re

I think we should just list these as normal keyword arguments, i.e.:

uint : bool, optional
    Interpret signed ...

(hey @pllim I found an easy io.fits issue!)

[bsipocz]

(hey @pllim I found an easy io.fits issue!)

famous last words...

[pllim]

You think it is easy... We'll see... :wink: