stmhal: Add documentation in comments, and script to generate HTML.

Decided to write own script to pull documentation from comments in C code.
Style for writing auto generated documentation is: start line with ///
and then use standard markdown to write the comment.  Keywords
recognised by the scraper begin with backslash.  See code for examples.

Running: python gendoc.py modpyb.c accel.c adc.c dac.c extint.c i2c.c
led.c pin.c rng.c servo.c spi.c uart.c usrsw.c, will generate a HTML
structure in gendoc-out/.

gendoc.py is crude but functional.  Needed something quick, and this was
it.
stmhal: Oops, typo.
nlr.h: As we treat all warnings as errors, can't use #warning.

And this is not good.
py: If setjmp NLR implementation is forced, omit native versions.
py: Abstract no-return attribute for functions a bit.
objfun: Factor out function to report positional args mismatch.