Metrics docs: update per TW review - GC64 note

Author: andreyaksenov

doc/book/admin/monitoring/alerting.rst

@@ -28,9 +28,11 @@ Prometheus configuration.
 Lua memory
 """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
 
-The Lua memory is limited to 2 GB per instance. Monitoring ``tnt_info_memory_lua``
-metric may prevent memory overflow and detect the presence of bad Lua code
-practices.
+Monitoring ``tnt_info_memory_lua`` metric may prevent memory overflow and detect the presence of bad Lua code practices.
+
+..  NOTE::
+
+    The Lua memory is limited to 2 GB per instance if Tarantool doesn't have the GC64 mode enabled for LuaJIT.
 
 ..  code-block:: yaml
 

doc/book/admin/monitoring/metrics_reference.rst

@@ -49,7 +49,6 @@ The following metrics provide a picture of memory usage by the Tarantool process
                 the vinyl page index, and the vinyl bloom filters.
         *   -   ``tnt_info_memory_lua``
             -   Number of bytes used for the Lua runtime.
-                The Lua memory is limited to 2 GB per instance.
                 Monitoring this metric can prevent memory overflow.
         *   -   ``tnt_info_memory_net``
             -   Number of bytes used for network input/output buffers.