makes it sound less misleading

actionpack/lib/action_dispatch/http/request.rb

@@ -209,8 +209,8 @@ def content_length
     end
 
     # Returns true if the "X-Requested-With" header contains "XMLHttpRequest"
-    # (case-insensitive). All major JavaScript libraries send this header with
-    # every Ajax request.
+    # (case-insensitive), which may need to be manually added depending on the
+    # choice of JavaScript libraries and frameworks.
     def xml_http_request?
       @env['HTTP_X_REQUESTED_WITH'] =~ /XMLHttpRequest/i
     end