hwdb: say that settings can be also set to 0

hwdb: say that settings can be also set to 0

We recommend that users create overriddes. This creates the problem that there
is no syntax to unset a property. Thus, the user needs to just set the property
to "something else" in the override file. But then the blurb saying that
"VAR=1" (or "VAR=0" in some cases) is the only allowed value can be confusing.
Say that both 0 and 1 can be set, since this documentation is also intended
for end users.

In our files, we generally don't want the override values anywhere. But we
have a test which checks the rvalue, which should be enough.