cmake: improve documentation for CAPNP_GENERATE_CPP
cmake: improve documentation for CAPNP_GENERATE_CPP

pjulien/capnproto,mrdomino/capnproto,jparyani/capnproto,jparyani/capnproto,jparyani/capnproto,mologie/capnproto,mrdomino/capnproto,pjulien/capnproto,mrdomino/capnproto,pjulien/capnproto,mologie/capnproto,mologie/capnproto,hntrmrrs/capnproto,hntrmrrs/capnproto,hntrmrrs/capnproto