Updates description of box_index.parts with new methods (#5126)

* Updates description of ```box_index.parts``` with new methods
* Since version 3.0.0 the ```box_index.parts``` can use methods of ```key_def```

Fixes #3444

Author: AArdeev

doc/reference/reference_lua/box_index/parts.rst

@@ -1,14 +1,41 @@
 .. _box_index-parts:
 
 ===============================================================================
-index_object:parts
+index_object.parts
 ===============================================================================
 
 .. class:: index_object
 
     .. data:: parts
 
-        The index's key parts.
+    The index's key parts. 
+    Since version :doc:`3.0.0 </release/3.0.0>`, the ``index_object.parts`` can operate methods
+    :ref:`extract_key() <key_def-extract_key>`,
+    :ref:`compare() <key_def-compare>`,
+    :ref:`compare_with_key() <key_def-compare_with_key>`,
+    :ref:`merge() <key_def-merge>`.
+
+    **``index_object.parts`` example**
+
+    ..  code-block:: lua
+
+            box.schema.space.create('T')
+            i = box.space.T:create_index('I', {parts={3, 'string', 1, 'unsigned'}})
+            box.space.T:insert{1, 99.5, 'X', nil, 99.5}
+            i.parts:extract_key(box.space.T:get({'X', 1}))
+
+    **``key_def`` equivalent**
+
+        ..  code-block:: lua
+
+            key_def = require('key_def')
+            box.schema.space.create('T')
+            i = box.space.T:create_index('I', {parts={3, 'string', 1, 'unsigned'}})
+            box.space.T:insert{1, 99.5, 'X', nil, 99.5}
+            k = key_def.new(i.parts)
+            k:extract_key(box.space.T:get({'X', 1}))
+
+    The outcome of the methods calling is described in :ref:`key_def_object <key_def_object>`.
 
         :rtype: table