Add property documentation

Add usergroupsFieldAllow and usergroupsFieldDisallow property documentation

Author: Jako

en/extras/login/login.register.md

@@ -49,6 +49,8 @@ Register has some default properties packaged into it. They are:
 | useExtended                       | Whether or not to set any extra fields in the form to the Profiles extended field. This can be useful for storing extra user fields.                                                                                                       | 1                   |
 | usergroups                        | Optional. A comma-separated list of User Group names or IDs to add the newly-registered User to.                                                                                                                                           |                     |
 | usergroupsField                   | Optional. The name of the field to use for the new User's usergroups, eg: using checkbox or radio input.                                                                                                                                   |                     |
+| usergroupsFieldAllow              | Optional. A comma-separated list of User Group names or IDs the newly-registered User can be added to. Only used if this value and the usergroupsField property is not blank.                                                          |                     |
+| usergroupsFieldDisallow           | Optional. A comma-separated list of User Group names or IDs to prevent the newly-registered User from being added to. Only used if this value and the usergroupsField property is not blank.                                          |                     |
 | usernameField                     | The name of the field to use for the new User's username.                                                                                                                                                                                  | username            |
 | validate                          | A comma-separated list of fields to validate, with each field name as name:validator (eg: username:required,email:required). Validators can also be chained, like email:email:required. This property can be specified on multiple lines.  |                     |
 | validatePassword                  | Whether or not to validate the sent password when registering. Recommended to leave this Yes unless you are generating your own password in a hook.                                                                                        | 1                   |