Fix javadoc for the NetworkRequest version of requestNetwork.

The documentation for this method says that the request can be
released using releaseNetworkRequest, but that's not true.
releaseNetworkRequest only takes a PendingIntent, and can only be
used to release a request filed with a PendingIntent.

Fix the docs to say that the request needs to be released using
unregisterNetworkCallback.

Change-Id: If044fd2d463ab8d09874172d5d56946251057a3c