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

[https://www.arduino.cc/reference/en/language/functions/communication/serial/availableforwrite/](url) 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

Start with the availableForWrite() reference page at https://www.arduino.cc/reference/en/language/functions/communication/serial/availableforwrite/ and review its current description. Done means the page states the returned value's datatype and tells readers where to find that information, so they can choose an appropriate variable for non-blocking writes.

Written by the indexing model from the issue text.