Update documentation for display brightness.

PowerHalService relies on VHAL to use property change event to
communicate the display brightness. This CL updates the doc
to be explicit about this requirement.

Furthermore, getting PER_DISPLAY_BRIGHTNESS without specifying
which port to get is confusing, so this CL also updates the doc.

Test: None, no code change
Bug: 336831738
Change-Id: Id4d0e7ff5fc065a53a38e831810470469563c3e6