Clarify WebResourceRequest.isForMainFrame docs.

Make it more explicit that isForMainFrame() returns true only for the
actual main frame document, and not for any subresources, even if they
are subresources of the main frame.

Test: make ds-docs
Change-Id: I942cbe6a8f50d2f3af00b99b14a8503c8c4556de

core/java/android/webkit/WebResourceRequest.java

@@ -32,10 +32,10 @@ public interface WebResourceRequest {
     Uri getUrl();
 
     /**
-     * Gets whether the request was made for the main frame.
+     * Gets whether the request was made in order to fetch the main frame's document.
      *
-     * @return whether the request was made for the main frame. Will be {@code false} for iframes,
-     *         for example.
+     * @return whether the request was made for the main frame document. Will be
+     *         {@code false} for subresources or iframes, for example.
      */
     boolean isForMainFrame();