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.)

Freshened patch,

Also adding the one-byte fix from #20201 to the rollup patch here.  (Not bothering to upload a fresh patch because it is literally one byte, fixing a misspelled variable name.)

I goofed, I do have some additional fixes.  Here's everything.

Changes since patch #1:
* Added 'converter' argument to 'object' converter.
* Fixed misspelled variable name (see #20201).
* Several documentation updates.

Third patch, with these new features:

* Relaxed rules regarding c_default and py_default; they can now be
  used even when no default value is supplied.
* Added bool return converter.
* Made one fail() call easier to read, through the power of str.format().
* More doc updates and fixes.

New changeset 3eee3967d03f by Larry Hastings in branch 'default':
Issue #20214: Fixed a number of small issues and documentation errors in
http://hg.python.org/cpython/rev/3eee3967d03f