Add some comments to IOThreadExtensionFunction explaining its intended use.

BUG=no
TEST=no


Review URL: http://codereview.chromium.org/7663019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@97081 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/extensions/extension_function.h

@@ -278,7 +278,12 @@ class UIThreadExtensionFunction : public ExtensionFunction {
   scoped_ptr<RenderViewHostTracker> tracker_;
 };
 
-// Extension functions that run on the IO thread.
+// Extension functions that run on the IO thread. This type of function avoids
+// a roundtrip to and from the UI thread (because communication with the
+// extension process happens on the IO thread). It's intended to be used when
+// performance is critical (e.g. the webRequest API which can block network
+// requests). Generally, UIThreadExtensionFunction is more appropriate and will
+// be easier to use and interface with the rest of the browser.
 class IOThreadExtensionFunction : public ExtensionFunction {
  public:
   IOThreadExtensionFunction();