fixes gh-1737 describe the new way to write 'parts' option while creating indexes

doc/reference/reference_lua/box_space/create_index.rst

@@ -152,6 +152,34 @@ On this page:
               name: primary
             ...
 
+        .. _index_parts_declaration_note:
+
+        ..  NOTE::
+
+            **Alternative way to declare index parts**
+
+            Before version :doc:`2.7.1 </release/2.7.1>`,
+            if an index consisted of a single part and had some options like
+            ``is_nullable`` or ``collation`` and its definition was written as
+
+            ..  code-block:: lua
+
+                my_space:create_index('one_part_idx', {parts = {1, 'unsigned', is_nullable=true}})
+
+            (with the only brackets) then options were ignored by Tarantool.
+
+            Since version :doc:`2.7.1 </release/2.7.1>` it is allowed to omit
+            extra braces in an index definition and use both ways:
+
+            ..  code-block:: lua
+
+                -- with extra braces
+                my_space:create_index('one_part_idx', {parts = {{1, 'unsigned', is_nullable=true}}})
+
+                -- without extra braces
+                my_space:create_index('one_part_idx', {parts = {1, 'unsigned', is_nullable=true}})
+
+
 .. _details_about_index_field_types:
 
 .. _box_space-index_field_types:

doc/release/2.7.1.rst

@@ -39,7 +39,8 @@ Core
 ~~~~
 
 -   Now it is allowed to define an index without extra braces when there
-    is only one part: ``parts = {field1, type1, ...}`` (gh-2866).
+    is only one part: ``parts = {field1, type1, ...}`` (gh-2866). Read more in
+    the :ref:`note about index parts declaration <index_parts_declaration_note>`.
 -   :ref:`Bitset index <indexes-bitset>` now supports the varbinary type (gh-5392).
 -   Index-related options now can’t be specified in their definition due
     to a more pedantic key-parts verification (gh-5473).