Provide links to Lua and LuaJIT data types

Resolves #1318

Author: patiencedaur

doc/reference/reference_lua/box_null.rst

@@ -6,6 +6,8 @@
 
 There are some major problems with using Lua **nil** values in tables.
 For example: you can't correctly assess the length of a table that is not a sequence.
+(Learn more about data types in `Lua <https://www.lua.org/manual/5.1/manual.html#2.2>`__
+and `LuaJIT <http://luajit.org/ext_ffi_semantics.html>`__.)
 
 **Example:**
 
@@ -45,7 +47,7 @@ without a value.
 Using box.NULL
 ~~~~~~~~~~~~~~
 
-``box.NULL`` is a value of the cdata type representing a NULL pointer.
+``box.NULL`` is a value of the `cdata <http://luajit.org/ext_ffi_semantics.html>`__ type representing a NULL pointer.
 It is similar to ``msgpack.NULL``, ``json.NULL`` and ``yaml.NULL``. So it is
 some not **nil** value, even if it is a pointer to NULL.
 

locale/ru/LC_MESSAGES/reference/reference_lua/box_null.po

@@ -5,11 +5,14 @@ msgstr "Константа `box.NULL`"
 msgid ""
 "There are some major problems with using Lua **nil** values in tables. For "
 "example: you can't correctly assess the length of a table that is not a "
-"sequence."
+"sequence. (Learn more about data types in `Lua <https://www.lua.org/manual/5.1/manual.html#2.2>`__ "
+"and `LuaJIT <http://luajit.org/ext_ffi_semantics.html>`__.)"
 msgstr ""
 "Имеется целый ряд серьезных проблем при использовании значения **nil** из "
 "Lua в таблицах. Например: вы не можете корректно оценить длину таблицы, не "
-"являющейся последовательностью."
+"являющейся последовательностью. (Узнайте больше о типах данных в `Lua "
+"<https://www.lua.org/manual/5.1/manual.html#2.2>`__ "
+"и `LuaJIT <http://luajit.org/ext_ffi_semantics.html>`__.)"
 
 msgid "**Example:**"
 msgstr "**Пример:**"
@@ -63,7 +66,7 @@ msgid "Using box.NULL"
 msgstr "Использование box.NULL"
 
 msgid ""
-"``box.NULL`` is a value of the cdata type representing a NULL pointer. It is"
+"``box.NULL`` is a value of the `cdata <http://luajit.org/ext_ffi_semantics.html>`__ type representing a NULL pointer. It is"
 " similar to ``msgpack.NULL``, ``json.NULL`` and ``yaml.NULL``. So it is some"
 " not **nil** value, even if it is a pointer to NULL."
 msgstr ""