docs: kdoc: don't reinvent string.strip()

docs: kdoc: don't reinvent string.strip()

process_proto_type() and process_proto_function() reinventing the strip()
string method with a whole series of separate regexes; take all that out
and just use strip().

The previous implementation also (in process_proto_type()) removed C++
comments *after* the above dance, leaving trailing whitespace in that case;
now we do the stripping afterward.  This results in exactly one output
change: the removal of a spurious space in the definition of
BACKLIGHT_POWER_REDUCED - see
https://docs.kernel.org/gpu/backlight.html#c.backlight_properties.

I note that we are putting semicolons after #define lines that really
shouldn't be there - a task for another day.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>