src/libpcp/src/derive_parser.y.in: add help text for event.* metrics
src/libpcp/src/derive_parser.y.in: add help text for event.* metrics

These ones are anonymous so we can't ask any PMDA to help provide the
help text ... weld it into libpcp.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp