datetime: describe relational operators

Follows up commit de58780
("Add a new section for the datetime module with the description of its main functions and methods").

Datetime comparison has been added in [1] (since 2.10.0-beta2),
interval comparison has been added in commit [2]
(since 2.11.0-rc1).

1. tarantool/tarantool@43e10ed
2. tarantool/tarantool@65a3c17

Folows up #2411

Author: ligurio

doc/reference/reference_lua/datetime.rst

@@ -964,3 +964,53 @@ The matrix of the ``subtraction`` operands eligibility and their result types:
             -   unsupported
             -   interval
             -   interval
+
+.. _interval_comp:
+
+Datetime and interval comparison
+--------------------------------
+
+If you need to compare the ``datetime`` and ``interval`` object values, you can use standard Lua relational operators: ``==`` , ``~=``, ``>``, ``<``, ``>=`` and ``<=`` using overloaded metamethods (``__eq``, ``__lt``, ``__le``).
+
+Support of relational operators for ``interval`` objects has been added since: :doc:`2.11.0 </release/2.11.0>`.
+
+        **Example 1:**
+
+        ..  code-block:: tarantoolsession
+
+            tarantool> dt1 = datetime.now()
+            ---
+            ...
+
+            tarantool> dt2 = datetime.now()
+            ---
+            ...
+
+            tarantool> dt1 == dt2
+            ---
+            - false
+            ...
+
+            tarantool> dt1 < dt2
+            ---
+            - true
+            ...
+
+        **Example 2:**
+
+        ..  code-block:: tarantoolsession
+
+            tarantool> iv1 = datetime.interval.new({month = 1})
+            ---
+            ...
+
+            tarantool> iv2 = datetime.interval.new({month = 2})
+            ---
+            ...
+
+            tarantool> iv1 < iv2
+            ---
+            - true
+            ...
+
+            tarantool>