reflect/protodesc: improve documentation
reflect/protodesc: improve documentation

Change-Id: Id1c0e0e702a7b0eb011c71de23cd5be76345d741
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/220341
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go