Improve DocumentsProvider and contract docs.

Bug: 10567557
Change-Id: I71e7f7ad172e9986e58b9c9533b1c948c5530397
diff --git a/core/java/android/content/ContentProvider.java b/core/java/android/content/ContentProvider.java
index 461dc1d..a9d0559 100644
--- a/core/java/android/content/ContentProvider.java
+++ b/core/java/android/content/ContentProvider.java
@@ -1078,6 +1078,7 @@
      * @see #openAssetFile(Uri, String)
      * @see #openFileHelper(Uri, String)
      * @see #getType(android.net.Uri)
+     * @see ParcelFileDescriptor#parseMode(String)
      */
     public ParcelFileDescriptor openFile(Uri uri, String mode)
             throws FileNotFoundException {
@@ -1147,6 +1148,7 @@
      * @see #openAssetFile(Uri, String)
      * @see #openFileHelper(Uri, String)
      * @see #getType(android.net.Uri)
+     * @see ParcelFileDescriptor#parseMode(String)
      */
     public ParcelFileDescriptor openFile(Uri uri, String mode, CancellationSignal signal)
             throws FileNotFoundException {