Clarified conditions that require client-side unbindService() call.

The documentation for android.app.Service includes an example from
LocalServiceActivities.java. This example now better shows and
explains the distinction between binding and connecting. It also
shows the set of conditions that require the client to call
unbindService().

Test: make ds-docs -j16

Bug: 63118511
Change-Id: I1276de3f9421e13e1964039d37db9adc87f3039f