Docs: Note that a Surface acts like a weak reference

This causes frequent programming errors, when developers assume
that holding onto a Surface will keep its associated SurfaceTexture,
ImageReader, etc, also alive.

Bug: 31551063
Test: m offline-sdk-docs, manual viewing of result
Change-Id: I5fb5bb3e3c80c7d5d735417b1697e0fe9a62fc46
diff --git a/core/java/android/view/Surface.java b/core/java/android/view/Surface.java
index f92d83a..9f46f3f 100644
--- a/core/java/android/view/Surface.java
+++ b/core/java/android/view/Surface.java
@@ -33,6 +33,18 @@
 
 /**
  * Handle onto a raw buffer that is being managed by the screen compositor.
+ *
+ * <p>A Surface is generally created by or from a consumer of image buffers (such as a
+ * {@link android.graphics.SurfaceTexture}, {@link android.media.MediaRecorder}, or
+ * {@link android.renderscript.Allocation}), and is handed to some kind of producer (such as
+ * {@link android.opengl.EGL14#eglCreateWindowSurface(android.opengl.EGLDisplay,android.opengl.EGLConfig,java.lang.Object,int[],int) OpenGL},
+ * {@link android.media.MediaPlayer#setSurface MediaPlayer}, or
+ * {@link android.hardware.camera2.CameraDevice#createCaptureSession CameraDevice}) to draw
+ * into.</p>
+ *
+ * <p><strong>Note:</strong> A Surface acts like a
+ * {@link java.lang.ref.WeakReference weak reference} to the consumer it is associated with. By
+ * itself it will not keep its parent consumer from being reclaimed.</p>
  */
 public class Surface implements Parcelable {
     private static final String TAG = "Surface";
diff --git a/media/java/android/media/ImageReader.java b/media/java/android/media/ImageReader.java
index 2cbeb3a..ed71849 100644
--- a/media/java/android/media/ImageReader.java
+++ b/media/java/android/media/ImageReader.java
@@ -241,6 +241,10 @@
      * same {@link Surface} can be reused with a different API once the first source is
      * disconnected from the {@link Surface}.</p>
      *
+     * <p>Please note that holding on to the Surface object returned by this method is not enough
+     * to keep its parent ImageReader from being reclaimed. In that sense, a Surface acts like a
+     * {@link java.lang.ref.WeakReference weak reference} to the ImageReader that provides it.</p>
+     *
      * @return A {@link Surface} to use for a drawing target for various APIs.
      */
     public Surface getSurface() {