protoreflect: adjust documentation
protoreflect: adjust documentation

After a header, there needs to be a paragraph in order for godoc
to recognize the header as a header.

Change-Id: I6c814b24dbb5eb2d5efcb0040556521c05587393
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/219140
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go