Adds description of `lua_call option specifics

* ``lua_call`` option allows the specified user to perform the specified lua function on the instance
* permissions can be granted to any user registered on the instance
* ``lua_call: [all]`` grants access to all global Lua functions except built-in ones
* grants are valid until instance is rebooted
* Fixes 4552

Author: AArdeev

doc/code_snippets/snippets/config/instances.enabled/lua_call/config.yaml

@@ -0,0 +1,7 @@
+credentials:
+  users:
+    alice:
+      privileges:
+        - permissions: [execute]
+          lua_call: [my_func]
+          

doc/reference/configuration/configuration_reference.rst

@@ -3220,6 +3220,26 @@ The ``lua`` section outlines the configuration parameters related to the Lua env
     | Default: 2147483648 (2GB)
     | Environment variable: TT_LUA_MEMORY
 
+.. _configuration_reference_lua_call:
+
+.. confval:: lua_call
+
+    Since version :doc:`3.3.0 </release/3.3.0>`, the ``lua_call`` option allows the specified user to perform the specified lua function on 
+    the instance during runtime.
+
+    Via the ``lua_call`` option, one can grant permissions to the function to any user registered on the instance. 
+
+    Note that the special option ``lua_call: [all]`` is also supported, granting access to all global Lua functions except built-in ones,   
+    bypassing database restrictions.
+
+    After the instance is rebooted, permissions defined via the ``lua_call`` options are reset to the values stored in the database.
+
+    Example to grant custom function to the 'alice' user:
+
+    ..  literalinclude:: /code_snippets/snippets/config/instances.enabled/lua_call/config.yaml
+        :language: yaml
+        :dedent:
+
 ..  _configuration_reference_memtx:
 
 memtx