Peter M. Groen / libmodbus

Thanks to Karl Palsson.

Inspired by
https://github.com/remakeelectric/libmodbus/commit/56b2526f5140a7e7efd0a59d5f2e5fb200f5a632

Change API of function for libmodbus v3.2.0

The report slave id function returns the number of bytes as the _return_
value, and does not include it as the first byte in the dest buffer.

Update documentation to reflect reality.  There's no missing
functionality, just slightly confusing documentation.