Update index.md

Added line to indicate a good location for custom exception classes inline with https://github.com/moodle/moodle/blob/main/lib/apis.json#L107

Author: mhughes2k

general/development/policies/codingstyle/index.md

@@ -2054,7 +2054,9 @@ We have a set of custom exception classes. The base class is moodle_exception. Y
 
 To get the full list of exception types, search for the regular expression 'class +\w+_exception +extends' or ask your IDE to list all the subclasses of moodle_exception.
 
-Where appropriate, you should create new subclasses of moodle_exception for use in your code.
+Where appropriate, you should create new subclasses of moodle_exception for use in your code. 
+
+If you create a custom exception class it *may* live in the `classes/exception/` directory, and be namespaced in `<plugin>/exception/`
 
 A few notable exception types: