Miscellaneous Argument Clinic fixes:
  * Added default value type verification for almost all return converters.
  * Added return converters for float, dobule, unsigned long,
    unsigned int, short, and unsigned short.
  * Py_buffer converter improvements:
      * Now allow exactly one value as a default value: None.
      * The default value in C only needs two fields initialized.
      * The cleanup call to PyBuffer_Release should be based on
        ".obj", not ".buf".
As well as a bunch of documentation fixes.

I'd love a review, but I'm pretty confident it's all okay so I may just check it in.  (The only code changes are in clinic.py so it's not going to break the world.)