Add javadoc for PEEK_HEIGHT_AUTO

Bug: 30088520
Change-Id: I31dccb6e56f527989cdb36266b7f76887f2ff32c
diff --git a/design/src/android/support/design/widget/BottomSheetBehavior.java b/design/src/android/support/design/widget/BottomSheetBehavior.java
index 20b4b74..1337352 100644
--- a/design/src/android/support/design/widget/BottomSheetBehavior.java
+++ b/design/src/android/support/design/widget/BottomSheetBehavior.java
@@ -110,6 +110,9 @@
 
     /**
      * Peek at the 16:9 ratio keyline of its parent.
+     *
+     * <p>This can be used as a parameter for {@link #setPeekHeight(int)}.
+     * {@link #getPeekHeight()} will return this when the value is set.</p>
      */
     public static final int PEEK_HEIGHT_AUTO = -1;