- Add a link field to a content type
- Provide custom help text
- Disable the link text field
- Save
The custom description is not used. See screenshots (it is still showing "This must be an external URL such as http://example.com.").
It seems that the custom help text is only applied to the link text component. If you set the field to only display the web address part, then the help text does not appear anywhere.
Suggested solution
Help text should apply to the whole fieldset. There should be dedicated overridable help text for each of the web address and link text parts.