Allow for multiple programs to watch for changes in the state
of input devices connected to the system. Previously only one program
at a time could watch input devices. While this functionality was not
implemented in BeOS R5, it was at least documented in the BeBook.
Also added some API documentation where necessary for the function
and related constants.
Change-Id: Icd927998cffcab212bb63bcf10c64c620e9da9a2
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3872
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>
