docs: Fixing details on when clients unbind from Bound Service

Docs were slightly contradictory on when clients should unbind.
Changed a few sections to point to one "source of truth" section
that explains the appropriate times to call unbindService().
Updated with corrections to issues found by Andrew and Dave.

Bug: 8135078
Change-Id: If8afb998cbd2efceef075ee6633cf0744c50d1df