Recommend all four payment request options should be used

It appeared to me that none of the payment request option really makes sense when used alone. Using both bitcoin: links, QR Codes and payment protocol at the same time in a payment request, and maybe providing fallback plain text request, should probably be promoted as a good practice.

The payment protocol wasn't mentioned and was kind of isolated like a separate payment request mechanism according to the title, while it is actually used inside a bitcoin: link in a backward-compatible way.