Fixes gh-766 Incomplete space/index iterator documentation

Author: pgulutzan

doc/1.10/book/box/box_index.rst

@@ -165,6 +165,10 @@ Below is a list of all ``box.index`` functions and members.
         <c_lua_tutorial-indexed_pattern_search>` shows one way that iterators
         and yields can be used together.
 
+        For information about iterators' internal structures see
+        `"Under the Hood" <https://luafun.github.io/under_the_hood.html>`_
+        in the the Lua functional documentation.
+
         :param index_object index_object: an :ref:`object reference
                                           <app_server-object_reference>`.
         :param scalar/table key: value to be matched against the index key,

doc/1.10/book/box/box_space.rst

@@ -1033,6 +1033,10 @@ Below is a list of all ``box.space`` functions and members.
         instead of "equal to"), see the later section :ref:`index_object:pairs
         <box_index-index_pairs>`.
 
+        For information about iterators' internal structures see
+        `"Under the Hood" <https://luafun.github.io/under_the_hood.html>`_
+        in the the Lua functional documentation.
+
         **Example:**
 
         .. code-block:: tarantoolsession