Move misplaced RTSP doc.

#minor-release

PiperOrigin-RevId: 373175041

docs/rtsp.md

+---
+title: RTSP
+---
+
+{% include_relative _page_fragments/supported-formats-rtsp.md %}
+
+## Using MediaItem ##
+
+To play an RTSP stream, you need to depend on the RTSP module.
+
+~~~
+implementation 'com.google.android.exoplayer:exoplayer-rtsp:2.X.X'
+~~~
+{: .language-gradle}
+
+You can then create a `MediaItem` for an RTSP URI and pass it to the player.
+
+~~~
+// Create a player instance.
+SimpleExoPlayer player = new SimpleExoPlayer.Builder(context).build();
+// Set the media item to be played.
+player.setMediaItem(MediaItem.fromUri(rtspUri));
+// Prepare the player.
+player.prepare();
+~~~
+{: .language-java}
+
+
+## Using RtspMediaSource ##
+
+For more customization options, you can create an `RtspMediaSource` and pass it
+directly to the player instead of a `MediaItem`.
+
+~~~
+// Create an RTSP media source pointing to an RTSP uri.
+MediaSource mediaSource =
+    new RtspMediaSource.Factory()
+        .createMediaSource(MediaItem.fromUri(rtspUri));
+// Create a player instance.
+SimpleExoPlayer player = new SimpleExoPlayer.Builder(context).build();
+// Set the media source to be played.
+player.setMediaSource(mediaSource);
+// Prepare the player.
+player.prepare();
+~~~
+{: .language-java}
+
+## Using RTSP behind NAT ##
+
+ExoPlayer uses UDP as the default protocol for RTP transport.
+
+When streaming RTSP behind a NAT layer, the NAT might not be able to forward the
+incoming RTP/UDP packets to the device. This occurs if the NAT lacks the
+necessary UDP port mapping. If ExoPlayer detects there have not been incoming
+RTP packets for a while and the playback has not started yet, ExoPlayer tears
+down the current RTSP playback session, and retries playback using RTP-over-RTSP
+(transmitting RTP packets using the TCP connection opened for RTSP).