doc extension concerning format_as and general implementations

[rimathia]

add a sentence about the possible interaction of format_as and general implementations

[rimathia]

Follow up on #4852.
My point here is only to have a hint somewhere close to format_as about the possible interaction with general implementations. I'm not very confident I've found the best wording, I trust that you will adjust as you see fit.

[vitaut]

I think something like this would be more precise:

format_as cannot be used when a type also matches another formatter specialization, such as the range formatter, because the specializations would be ambiguous. Disable the conflicting specialization, if possible, or provide an explicit formatter specialization instead.