Add description of privileges (#12021)

* Add description of privileges

* Restructure Privileges section based on feedback

* Suggestion by sfan5

Co-authored-by: sfan5 <sfan5@live.de>

* Suggestion by sfan5

Co-authored-by: sfan5 <sfan5@live.de>

* Incorporate comments by sfan5

Co-authored-by: sfan5 <sfan5@live.de>

doc/lua_api.txt

@@ -4692,7 +4692,92 @@ Spawn a small apple tree:
     minetest.spawn_tree(pos,apple_tree)
 
 
+Privileges
+==========
 
+Privileges provide a means for server administrators to give certain players
+access to special abilities in the engine, games or mods.
+For example, game moderators may need to travel instantly to any place in the world, 
+this ability is implemented in `/teleport` command which requires `teleport` privilege.
+
+Registering privileges
+----------------------
+
+A mod can register a custom privilege using `minetest.register_privilege` function 
+to give server administrators fine-grained access control over mod functionality.
+
+For consistency and practical reasons, privileges should strictly increase the abilities of the user.
+Do not register custom privileges that e.g. restrict the player from certain in-game actions.
+
+Checking privileges
+-------------------
+
+A mod can call `minetest.check_player_privs` to test whether a player has privileges 
+to perform an operation.
+Also, when registering a chat command with `minetest.register_chatcommand` a mod can
+declare privileges that the command requires using the `privs` field of the command
+definition.
+
+Managing player privileges
+--------------------------
+
+A mod can update player privileges using `minetest.set_player_privs` function.
+Players holding the `privs` privilege can see and manage privileges for all
+players on the server.
+
+A mod can subscribe to changes in player privileges using `minetest.register_on_priv_grant`
+and `minetest.register_on_priv_revoke` functions.
+
+Built-in privileges
+-------------------
+
+Minetest includes a set of built-in privileges that control capabilities
+provided by the Minetest engine and can be used by mods:
+
+  * Basic privileges are normally granted to all players:
+      * `shout`: can communicate using the in-game chat.
+      * `interact`: can modify the world by digging, building and interacting
+        with the nodes, entities and other players. Players without the `interact`
+        privilege can only travel and observe the world.
+
+  * Advanced privileges allow bypassing certain aspects of the gameplay:
+      * `fast`: can use "fast mode" to move with maximum speed.
+      * `fly`: can use "fly mode" to move freely above the ground without falling.
+      * `noclip`: can use "noclip mode" to fly through solid nodes (e.g. walls).
+      * `teleport`: can use `/teleport` command to move to any point in the world.
+      * `creative`: can access creative inventory.
+      * `bring`: can teleport other players to oneself.
+      * `give`: can use `/give` and `/giveme` commands to give any item
+        in the game to oneself or others.
+      * `settime`: can use `/time` command to change current in-game time.
+      * `debug`: can enable wireframe rendering mode.
+
+  * Security-related privileges:
+      * `privs`: can modify privileges of the players using `/grant[me]` and
+        `/revoke[me]` commands.
+      * `basic_privs`: can grant and revoke basic privileges as defined by
+        the `basic_privs` setting.
+      * `kick`: can kick other players from the server using `/kick` command.
+      * `ban`: can ban other players using `/ban` command.
+      * `password`: can use `/setpassword` and `/clearpassword` commands
+        to manage players' passwords.
+      * `protection_bypass`: can bypass node protection. Note that the engine does not act upon this privilege,
+        it is only an implementation suggestion for games.
+
+  * Administrative privileges:
+      * `server`: can use `/fixlight`, `/deleteblocks` and `/deleteobjects`
+        commands. Can clear inventory of other players using `/clearinv` command.
+      * `rollback`: can use `/rollback_check` and `/rollback` commands.
+
+Related settings
+----------------
+
+Minetest includes the following settings to control behavior of privileges:
+
+   * `default_privs`: defines privileges granted to new players.
+   * `basic_privs`: defines privileges that can be granted/revoked by players having
+    the `basic_privs` privilege. This can be used, for example, to give 
+    limited moderation powers to selected users.
 
 'minetest' namespace reference
 ==============================