Clarify JsniCollector.SourceCache comment.

Review by: jat (desk)

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@6616 8db76d5a-ed1c-0410-87a9-c151d255dfc7
diff --git a/dev/core/src/com/google/gwt/dev/javac/JsniCollector.java b/dev/core/src/com/google/gwt/dev/javac/JsniCollector.java
index e49d015..4d04213 100644
--- a/dev/core/src/com/google/gwt/dev/javac/JsniCollector.java
+++ b/dev/core/src/com/google/gwt/dev/javac/JsniCollector.java
@@ -133,7 +133,8 @@
   }
 
   /**
-   * Just a little class to grab source only once for a compilation unit.
+   * Lazily grab source only for compilation units which actually contain native
+   * methods, then cache it.
    * 
    * TODO: parse JSNI <i>during</i> JDT's cycle to avoiding having to read from
    * disk twice.