Add error codes documentation

lib/constants.py
* add to each CV_E* tuple the documentation of the error code
* add the DOCUMENTED_CONSTANTS constant for the doc preprocessor

autotools/docpp
* add a new directive class CONSTANTS_<kind>, that gets data from
  constants.DOCUMENTED_CONSTANTS

lib/cmdlib.py
* modify the code that unpacked the CV_E* tuples to ignore the
  documentation parameter

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>