docs/extensions/helptext.md

+# Adding pop-up help text in your extension
+
+In addition to simply adding help text for form fields that you have added in your extension as usual, you can also add to or replace help text for existing form fields in core. To do so, simply add a file in the same directory as the hlp file is found in core, with the name `helpfilename.extra.hlp`.
+
+If you want to append your help text to the existing text, add the htxt to your extra help file as normal. If you want to replace the existing text with your own text, add `override=1` to your htxt, for example: `{htxt id="field_name_id" override=1}...`. This works for both the content of the help text and the title.
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