Address API council feedback for
WifiP2pManager#discoverPeers.

Comments include:
 - Specify which discoverPeers method is linked
   in the documentation.
 - Explain the conditional permission requirement.
 - Rename the method to startPeerDiscovery.
 - Require the discovery config to be @NonNull.

Note that the explanation for the permissions
triggers a lint warning.

Bug: 324599086
Test: atest ConcurrencyTest#testStartPeerDiscovery
Change-Id: I4a80c5b3a397b707e37ee95913518c726433bad4