Formatting changes + clarification that most functionality is functions

I think we are on the same page here, but to confirm:

+ There is one function provided by the ``CCDData`` class, ``fromFITS``, which is denoted in the API doc as ``cddproc.CCDData.fromFITS``.
+ The rest of the functions, so far, are provided by the ``ccdproc`` package rather than being methods of a class; ``subtract_overscan`` is an example, and is denoted by ``ccdproc.subtract_overscan``

I believe this is consistent with the discussion to date at https://github.com/astropy/astropy-api/pull/8