I think this is what the Manual is saying:
By default, forum staff (Moderators and Administrator) can edit a user's post and not have the post marked as having been edited. And the edited post will remain open for further editing by the original poster, for as long as set by $useredit in setup_options.php.
To mark a post as having been edited and locked by a forum staff, the staff member needs to check the "Edit" box after he/she edits a post; please note, on an actual miniBB installation, the mentioned "Edit" box is displayed with a strike-through.
Instead of the default setup, as described in the previous two paragraphs, the Administrator has the option to implement the "Mark as edited and lock post" option for forum staff to use.
"Mark as edited and lock post" is essentially forum staff saying: "label this post as having been edited by staff, and lock the post so that its original author cannot erase the changes."
The "Mark as edited and lock post" option is installed by adding the following line in the appropriate language pack: $l_editLock='Mark as edited and lock post';
Once installed, the "Mark as edited and lock post" option will override miniBB's default, or any previously set edit-related settings.
All the best -