This form allows you to directly edit the XML representation of the field list. This makes it possible to add annotations for fieldsets, validation and widgets. See the Dexterity XML section of the Dexterity Developer Manual for details.
Ideally a content expert would complete the process I started and which others, including @MrTango, have been working on. Then we’d update those links in the control panel and call it done.
Until then, we can do one of two things:
Create Sphinx reredirect from https://6.docs.plone.org/external/plone.app.dexterity/docs/reference/dexterity-xml.html to https://5.docs.plone.org/external/plone.app.dexterity/docs/reference/dexterity-xml.html, and another from https://6.docs.plone.org/external/plone.app.dexterity/docs/index.html to https://5.docs.plone.org/external/plone.app.dexterity/docs/index.html. I'd accept a pull request, as well as an issue to update it when the Plone 6 docs for Dexterity are in place.
Create a PR to update the links in the control panel to point to the v5 docs, as well as an issue to update them when the Plone 6 docs for Dexterity are in place.