WifiScanner: Enhance doc for setScanningEnabled API

The existing WifiScanner.setScanningEnabled() API is invoked by
ActiveModeWarden after any client iface state changes. Documenting this
behavior explicitly (i.e enable needs to be invoked multiple time if
there are multiple STA's brought up).

Bug: 140111024
Test: None
Change-Id: Ie377a9b57119cb7389df60294fc2111ec654fd38
1 file changed