Adds new methods to index_object_parts

Since 3.1.0 4 new ```key_def``` methods are applicable to ```index_object.parts```
*```validate_key()```
*```validate_full_key```
*```validate_tuple()```
*```compare_keys()```
Functionally methods are equivalent to their description in ```key_def```
Fixes #5139

Author: AArdeev

doc/reference/reference_lua/box_index/parts.rst

@@ -15,6 +15,12 @@ index_object.parts
     :ref:`compare_with_key() <key_def-compare_with_key>`,
     :ref:`merge() <key_def-merge>`.
 
+    Since version :doc:`3.1.0 </release/3.1.0>`, the ``index_object.parts`` can operate methods
+    :ref:`validate_key() <key_validate_key>`,
+    :ref:`validate_full_key() <key_validate_full_key>`,
+    :ref:`validate_tuple() <key_validate_tuple>`,
+    :ref:`compare_keys() <key_compare_keys>`.
+
     **``index_object.parts`` example**
 
     ..  code-block:: lua