1. As was discussed, these definitions should go into
src/include/libpq/pqcomm.h, not a new file.
2. I would prefer an underscore after PgMsg, like PqMsg_DescribeRequest,
so it's easier to visually locate the start of the actual message name.
3. IMO, the names of the protocol messages in protocol.sgml are
canonical. Your patch appends "Request" and "Response" in cases where
that is not part of the actual name. Also, some messages are documented
to go both ways, so this separation doesn't make sense strictly
speaking. Please use the names as in protocol.sgml without augmenting them.