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

@@ -8,7 +8,15 @@ index_object:parts
 
     .. 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>`,
+    :ref:`totable() <key_def-totable>`.
+
+    The outcome of the methods calling is described in :ref:`key_def_object <key_def_object>`.
 
         :rtype: table