descriptors.py: Clarify use of negative values

The negative behavior is not obvious, document it in the descriptors.py file
such that people who edit it to add new devices know how it works.

lib/logitech/unifying_receiver/descriptors.py

@@ -76,6 +76,8 @@ def _D(name, codename=None, kind=None, registers=None, settings=None):
 
 # Some HID++1.0 registers and HID++2.0 features can be discovered at run-time,
 # so they are not specified here.
+# Registers are only supported for HID++ 1.0 devices. Specify a negative value
+# to blacklist that register (prevent Solaar from accessing the register)
 
 # Keyboards