Document and reference best practice for OHTTP Relay selection

[DanGould]

Our payjoin-cli implementation only uses one OHTTP Relay by configuration, and BullBitcoin Mobile selects one from those available at random

However, if a request fails due to an unresponsive OHTTP Relay, we've delivered no mechanism to retry on another relay. We ought to build this into payjoin-cli so that downstream implementations have a good idea of what they need to build for reliability and to preserve privacy.