Add Java doc comments on script group name reuse

b/21999787

Change-Id: Id9a7bad4ce91f3afdd82a1288cf94430f3aef9bb
(cherry picked from commit dbb6fd58a4b113f8761ce825ad5b91e50c170971)
diff --git a/rs/java/android/renderscript/ScriptGroup.java b/rs/java/android/renderscript/ScriptGroup.java
index 5a0897d..2878356 100644
--- a/rs/java/android/renderscript/ScriptGroup.java
+++ b/rs/java/android/renderscript/ScriptGroup.java
@@ -993,6 +993,8 @@
          *
          * @param name name for the script group. Legal names can only contain letters, digits,
          *        '-', or '_'. The name can be no longer than 100 characters.
+         *        Try to use unique names, to avoid name conflicts and reduce
+         *        the cost of group creation.
          * @param outputs futures intended as outputs of the script group
          * @return a script group
          */