abs() does not work on floats

I'm not sure how to rewrite the page, so I rather write it here as issue instead of pull-request.

abs() does not work on floats, because it returns an int. It does not say so in the documentation at https://github.com/arduino/reference-en/blob/master/Language/Functions/Math/abs.adoc

Apparently for floats there exists [fabs](https://www.mkssoftware.com/docs/man3/fabs.3.asp)(), but this is not written in the arduino ref docs.

Should this be added, or is there some reason for this information missing?