Clarify createPeriod javadoc.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=140202942

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

@@ -52,7 +52,9 @@ public interface MediaSource {
   void maybeThrowSourceInfoRefreshError() throws IOException;
   void maybeThrowSourceInfoRefreshError() throws IOException;
 
 
   /**
   /**
-   * Returns a {@link MediaPeriod} corresponding to the period at the specified index.
+   * Returns a new {@link MediaPeriod} corresponding to the period at the specified {@code index}.
+   * This method may be called multiple times with the same index without an intervening call to
+   * {@link #releasePeriod(MediaPeriod)}.
    *
    *
    * @param index The index of the period.
    * @param index The index of the period.
    * @param allocator An {@link Allocator} from which to obtain media buffer allocations.
    * @param allocator An {@link Allocator} from which to obtain media buffer allocations.