Clarify Use of Application Congestion Control

[martinduke]

sec 5.2.9
It would be helpful to say "Applications that neither prohibit nor require congestion should query the result and disable their own congestion control if present in the transport"

[gorryfair]

Interesting, but we need to be careful in what we say. It seems safe, but not performant to do CC at multiple layers, and if it's done at a lower layer performance might be improved by not replicating the function at a higher layer...

[tfpauly]

Link to UDP guidelines document.

Just mention that there are performance implications of multiple CC loops; don't just say "disable CC".