specs: explain touch behaviour for dependent devices

Dependent devices don't send touch events until the interaction is a true
touch interaction (i.e. doesn't just serve to move the pointer). Once that
happens, all touchpoints send touch events exclusively. Pointer movement
restarts once we're down to one touch that controls the pointer again.

For clients listening to touch events in addition to pointer events, this
also means that a two-finger tap looks identical to holding one finger down
and tapping with a second-finger. Both actions will result in short
TouchBegin/TouchEnd sequences for both fingers.

The above is the default behaviour we expect from touchpads, the protocol is
more generically worded to leave more room for drivers to decide when a
touch only controls the pointer and when it doesn't.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Chase Douglas <chase.douglas@canonical.com>

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