gmmproc: _WRAP_GERROR: Add documentation to the generated enum Code

* tools/pm/Output.pm: output_wrap_gerror(): Get the enum documentation from
the docs.xml file.
* tools/m4/gerror.m4: Include the documentation before 'enum Code' in
the generated code.

_WRAP_GERROR was overlooked when José Alburquerque implemented the
documentation of other enums (bug 544694).