Updated Frequently asked questions (markdown)

Frequently-asked-questions.md

@@ -21,4 +21,8 @@ Just add a rule at the end of your handlers list that matches everything:
 
     (r'.*', My404Handler)
 
-In `My404Handler`, use `self.set_status(404)` and then render a page as usual.  For non-404 errors, you can override `write_error` in your handler class.
+In `My404Handler`, use `self.set_status(404)` and then render a page as usual.  For non-404 errors, you can override `write_error` in your handler class.
+
+## Why don't tornado.escape.json_encode and json_decode pass unknown keyword arguments to the underlying implementation?  OR  Why don't json_encode and json_decode use [some faster json library] when available?
+
+These two common requests are mutually exclusive - one ties json_encode more closely to a specific implementation, and the other requires the underlying implementation be swappable.  In both cases I feel like the best approach is to leave tornado.escape.json_encode as a lowest-common-denominator json implementation that is available on all platforms where tornado runs. If you want more features that are provided by a specific json library, you should probably just import and use that library directly instead of using the tornado.escape version.