BIP 123: Clarify how used with non-Standards BIPs, and update list

Also: It's unclear where to draw the lines for API/RPC. The previous draft had Stratum (which is being removed here since there sadly isn't a BIP yet) as Applications and GBT as API/RPC, when both protocols serve the same fundamental purpose. Need to get this cleaned up to move forward on both it as well as BIP 2... @CodeShark

Specification