Improve LDAP group config documentation (#21227) (#26921)

backport #21227

author @svenseeberg

Co-authored-by: Sven Seeberg <mail@sven-seeberg.de>
Co-authored-by: Giteabot <teabot@gitea.io>

docs/content/usage/authentication.en-us.md

 
 - User Attribute in Group (optional)
 
-  - Which user LDAP attribute is listed in the group.
-  - Example: `uid`
+  - The user attribute that is used to reference a user in the group object.
+  - Example: `uid` if the group objects contains a `member: bender` and the user object contains a `uid: bender`.
+  - Example: `dn` if the group object contains a `member: uid=bender,ou=users,dc=planetexpress,dc=com`.
 
 - Group Attribute for User (optional)
-  - Which group LDAP attribute contains an array above user attribute names.
-  - Example: `memberUid`
+  - The attribute of the group object that lists/contains the group members.
+  - Example: `memberUid` or `member`
 
 ## PAM (Pluggable Authentication Module)
 

services/auth/source/ldap/README.md

   * Example: (|(cn=gitea_users)(cn=admins))
 
 * User Attribute in Group (optional)
-  * Which user LDAP attribute is listed in the group.
-  * Example: uid
+  * The user attribute that is used to reference a user in the group object.
+  * Example: uid if the group objects contains a member: bender and the user object contains a uid: bender.
+  * Example: dn if the group object contains a member: uid=bender,ou=users,dc=planetexpress,dc=com.
 
 * Group Attribute for User (optional)
-  * Which group LDAP attribute contains an array above user attribute names.
-  * Example: memberUid
+  * The attribute of the group object that lists/contains the group members.
+  * Example: memberUid or member
 
 * Team group map (optional)
   * Automatically add users to Organization teams, depending on LDAP group memberships.