Add some comments to IOThreadExtensionFunction explaining its intended use.

chrome/browser/extensions/extension_function.h

Index: chrome/browser/extensions/extension_function.h
diff --git a/chrome/browser/extensions/extension_function.h b/chrome/browser/extensions/extension_function.h
index f6ab934eaacc09a092097d2b818aaaf8d0033c38..6afbed885db27a2fc99c6b6129175c1e7a391523 100644
--- a/chrome/browser/extensions/extension_function.h
+++ b/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();