Adds ~/Documents/Arduino/ to a..._dist_dir_guesses

Adds ~/Documents/Arduino/ to arduino_dist_dir_guesses:
places to look for hardware containing boards.txt

I'm not sure arduino_dist_dir_guesses is the right place to add ~/Documents/Arduino/ to.
But it solves the problem that adding boards following the procedure described here:
https://learn.sparkfun.com/tutorials/pro-micro--fio-v3-hookup-guide/installing-mac--linux
will still not be found, even with skalniks multi-board branch.