Fixed #30600 -- Clarified that ValueError raised by converter.to_python() means no match.

docs/topics/http/urls.txt

@@ -146,7 +146,9 @@ A converter is a class that includes the following:
 
 * A ``to_python(self, value)`` method, which handles converting the matched
   string into the type that should be passed to the view function. It should
-  raise ``ValueError`` if it can't convert the given value.
+  raise ``ValueError`` if it can't convert the given value. A ``ValueError`` is
+  interpreted as no match and as a consequence a 404 response is sent to the
+  user.
 
 * A ``to_url(self, value)`` method, which handles converting the Python type
   into a string to be used in the URL.