Update documentation for VideoEncoder.Callback#onEncodedImage.

Allows assuming that the buffer is not accessed after the call returns.

Bug: b/72675429
No-Try: True
Change-Id: Iff4a05433c6eed6aefec49ce67486966b1ed882f
Reviewed-on: https://webrtc-review.googlesource.com/49161
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21940}

sdk/android/api/org/webrtc/VideoEncoder.java

@@ -122,7 +122,10 @@ public interface VideoEncoder {
   }
 
   public interface Callback {
-    /** Call to return an encoded frame. */
+    /**
+     * Call to return an encoded frame. It is safe to assume the byte buffer held by |frame| is not
+     * accessed after the call to this method returns.
+     */
     void onEncodedFrame(EncodedImage frame, CodecSpecificInfo info);
   }