Add some clarifications to MediaSource documentation

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177141094

library/core/src/main/java/com/google/android/exoplayer2/source/MediaSource.java

@@ -35,7 +35,8 @@ import java.io.IOException;
  *   player to load and read the media.</li>
  * </ul>
  * All methods are called on the player's internal playback thread, as described in the
- * {@link ExoPlayer} Javadoc.
+ * {@link ExoPlayer} Javadoc. They should not be called directly from application code. Instances
+ * should not be re-used, meaning they should be passed to {@link ExoPlayer#prepare} at most once.
  */
 public interface MediaSource {
 
@@ -150,6 +151,8 @@ public interface MediaSource {
 
   /**
    * Starts preparation of the source.
+   * <p>
+   * Should not be called directly from application code.
    *
    * @param player The player for which this source is being prepared.
    * @param isTopLevelSource Whether this source has been passed directly to
@@ -162,6 +165,8 @@ public interface MediaSource {
 
   /**
    * Throws any pending error encountered while loading or refreshing source information.
+   * <p>
+   * Should not be called directly from application code.
    */
   void maybeThrowSourceInfoRefreshError() throws IOException;
 
@@ -169,6 +174,8 @@ public interface MediaSource {
    * Returns a new {@link MediaPeriod} identified by {@code periodId}. This method may be called
    * multiple times with the same period identifier without an intervening call to
    * {@link #releasePeriod(MediaPeriod)}.
+   * <p>
+   * Should not be called directly from application code.
    *
    * @param id The identifier of the period.
    * @param allocator An {@link Allocator} from which to obtain media buffer allocations.
@@ -178,6 +185,8 @@ public interface MediaSource {
 
   /**
    * Releases the period.
+   * <p>
+   * Should not be called directly from application code.
    *
    * @param mediaPeriod The period to release.
    */
@@ -186,8 +195,7 @@ public interface MediaSource {
   /**
    * Releases the source.
    * <p>
-   * This method should be called when the source is no longer required. It may be called in any
-   * state.
+   * Should not be called directly from application code.
    */
   void releaseSource();