Improve camera getSupportedPreviewFormats javadoc.

bug:3463624
Change-Id: I4151fc7821e9648c6429acca53582f11f35f8813
diff --git a/core/java/android/hardware/Camera.java b/core/java/android/hardware/Camera.java
index aabe872..d5dadf1 100644
--- a/core/java/android/hardware/Camera.java
+++ b/core/java/android/hardware/Camera.java
@@ -1694,7 +1694,9 @@
         }
 
         /**
-         * Gets the supported preview formats.
+         * Gets the supported preview formats. {@link android.graphics.ImageFormat#NV21}
+         * is always supported. {@link android.graphics.ImageFormat#YV12}
+         * is always supported since API level 12.
          *
          * @return a list of supported preview formats. This method will always
          *         return a list with at least one element.