Documentation - add WAGTAILADMIN_RICH_TEXT_EDITORS reference (#9079)

- Mention settings on admin customisation page
- fixes #9070

docs/advanced_topics/customisation/page_editing_interface.md

@@ -90,6 +90,8 @@ The process for creating new features is described in the following pages:
 -   [](../../extending/rich_text_internals)
 -   [](../../extending/extending_draftail)
 
+You can also provide a setting for naming a group of rich text features. See [WAGTAILADMIN_RICH_TEXT_EDITORS](wagtailadmin_rich_text_editors).
+
 (rich_text_image_formats)=
 
 ### Image Formats in the Rich Text Editor