Update documentation for startScan

With change c4a1e1, unfiltered BLE scans are stopped on screen off
and resumed when screen is turned back on. This is done to save power.
This change updates the documentation accordingly.

BUG: 62264269
Test: Documentation update.
Change-Id: I2d8e9f9f122f978c4d4f59d4139cb51cd4e4a123