WifiManager: update docs on setWifiEnabled

Update documentation to the setWifiEnabled call reflect what a return of
false would be for.  Note: False will be returned if the caller
requested to enable/disable wifi while in softap mode or airplane mode
is active.

Bug: 64374508
Test: checked generated docs
Change-Id: I1b1de84afee3844d326e7780a935a30d7f64d220