gmmproc: Tidy up the generation of enum docs

* tools/m4/gerror.m4: Add "/** " at the start of the documentation,
like enum.m4 does.
* tools/pm/DocsParser.pm: lookup_enum_documentation(): Don't add "/** "
which is deleted by the caller. Add "\n * " with the right indentation.
* tools/pm/Output.pm: output_wrap_enum(), output_wrap_enum_docs_only(),
output_wrap_gerror(): Don't delete "/** ". Don't change indentation.