Fix mistake when calling on_priv_grant/revoke, and document them (#6341)

builtin/game/register.lua

@@ -437,9 +437,9 @@ end

function core.run_priv_callbacks(name, priv, caller, method)
	if not core.registered_privileges[priv]["on_" .. method] or
			core.registered_privileges[priv]["on_" .. method](name, caller) then
			not core.registered_privileges[priv]["on_" .. method](name, caller) then
		for _, func in ipairs(core["registered_on_priv_" .. method]) do
			if not func(name, caller) then
			if not func(name, caller, priv) then
				break
			end
		end

doc/lua_api.txt

@@ -2447,6 +2447,14 @@ Call these functions only at load time!
* `minetest.register_on_item_eat(func(hp_change, replace_with_item, itemstack, user, pointed_thing))`
    * Called when an item is eaten, by `minetest.item_eat`
    * Return `true` or `itemstack` to cancel the default item eat response (i.e.: hp increase)
* `minetest.register_on_priv_grant(function(name, granter, priv))`
    * Called when `granter` grants the priv `priv` to `name`.
    * Note that the callback will be called twice if it's done by a player, once with granter being the player name,
      and again with granter being nil.
* `minetest.register_on_priv_revoke(function(name, revoker, priv))`
    * Called when `revoker` revokes the priv `priv` from `name`.
    * Note that the callback will be called twice if it's done by a player, once with revoker being the player name,
      and again with revoker being nil.

### Other registration functions
* `minetest.register_chatcommand(cmd, chatcommand definition)`
@@ -2464,8 +2472,9 @@ Call these functions only at load time!
         `granter_name` will be nil if the priv was granted by a mod.
    * `on_revoke(name, revoker_name)`: Called when taken from player `name` by `revoker_name`.
         `revoker_name` will be nil if the priv was revoked by a mod
    * Note that the above two callbacks will be called twice if a player is responsible - 
    * Note that the above two callbacks will be called twice if a player is responsible -
      once with the player name, and then with a nil player name.
    * Return true in the above callbacks to stop register_on_priv_grant or revoke being called.
* `minetest.register_authentication_handler(handler)`
    * See `minetest.builtin_auth_handler` in `builtin.lua` for reference