clarify blueprint 404 error handling in docs

docs/blueprints.rst

@@ -245,4 +245,22 @@ Here is an example for a "404 Page Not Found" exception::
     def page_not_found(e):
         return render_template('pages/404.html')
 
+Most errorhandlers will simply work as expected; however, there is a caveat
+concerning handlers for 404 and 405 exceptions.  These errorhandlers are only
+invoked from an appropriate ``raise`` statement or a call to ``abort`` in another
+of the blueprint's view functions; they are not invoked by, e.g., an invalid URL
+access.  This is because the blueprint does not "own" a certain URL space, so
+the application instance has no way of knowing which blueprint errorhandler it
+should run if given an invalid URL.  If you would like to execute different
+handling strategies for these errors based on URL prefixes, they may be defined
+at the application level using the ``request`` proxy object::
+
+    @app.errorhandler(404)
+    @app.errorhandler(405)
+    def _handle_api_error(ex):
+        if request.path.startswith('/api/'):
+            return jsonify_error(ex)
+        else:
+            return ex
+
 More information on error handling see :ref:`errorpages`.