Add missing docs for <field><permission> in XML schema
During discussion of permissions for Standalone over at https://github.com/civicrm/civicrm-core/pull/28877 it turns out there's a to-date undocumented feature in the XML schema that provides field-level access control by permission.
This hopes to document that.
This PR just adds in the features that I can observe examples of in Core. There may be other features (<and>
? nested <and>|<or>
?).