wifi: document UnsupportedOperationException for Passpoint related APIs

An UnsupportedOperationException will be thrown for Passpoint related
API calls if Passpoint is not enabled on the device (build config
config_wifi_hotspot2_enabled is set to false).

Bug: None
Test: None
Change-Id: Id0ba1f7b80091d55a839fa3eee36baf9b45b7f5f