Document Bitmap recycling in BitmapTransformation.

Fixes #422.

library/src/main/java/com/bumptech/glide/load/resource/bitmap/BitmapTransformation.java

@@ -68,6 +68,15 @@ public abstract class BitmapTransformation implements Transformation<Bitmap> {
      * result.
      *
      * <p>
+     *   The provided Bitmap, toTransform, should not be recycled or returned to the pool. Glide will automatically
+     *   recycle and/or reuse toTransform if the transformation returns a different Bitmap. Similarly implementations
+     *   should never recycle or return Bitmaps that are returned as the result of this method. Recycling or returning
+     *   the provided and/or the returned Bitmap to the pool will lead to a variety of runtime exceptions and drawing
+     *   errors. See #408 for an example. If the implementation obtains and discards intermediate Bitmaps, they may
+     *   safely be returned to the BitmapPool and/or recycled.
+     * </p>
+     *
+     * <p>
      *     outWidth and outHeight will never be {@link com.bumptech.glide.request.target.Target#SIZE_ORIGINAL}, this
      *     class converts them to be the size of the Bitmap we're going to transform before calling this method.
      * </p>