Reference of Serial.availableForWrite() does not state what returned value is nor where to find out.

[brewmanz]

https://www.arduino.cc/reference/en/language/functions/communication/serial/availableforwrite/ does not tell the user about the returned value:

• What its datatype is (signed or not, whether 8 or 16 or ... bits)
• Where to find out this information

I want to check that there is enough space for non-blocking writes; and so what type/size of variable do I need to use?
For i have an Arduino chip of very little brain, and long words or other datatypes bother me^.
Yes, I've since discovered that it's an int, but why not just tell people without making them search the source?

^ Apologies to A. A. Milne

[per1234]

This is a small piece of the big picture, which is that the essential documentation of parameter and return types is missing or incorrect throughout much of the reference: #15