Add tuple_object.format() reference (#4714)

Resolves #4301

Author: p7nov

doc/reference/reference_lua/box_tuple.rst

@@ -47,6 +47,9 @@ Below is a list of all ``box.tuple`` functions.
         *  - :doc:`./box_tuple/find`
            - Get the number of the first field/all fields matching the search value
 
+        *  - :doc:`./box_tuple/format`
+           - Get the format of a tuple
+
         *  - :doc:`./box_tuple/info`
            - Get information about the tuple
 
@@ -85,6 +88,7 @@ Below is a list of all ``box.tuple`` functions.
     box_tuple/field_name
     box_tuple/field_path
     box_tuple/find
+    box_tuple/format
     box_tuple/info
     box_tuple/next
     box_tuple/pairs

doc/reference/reference_lua/box_tuple/bsize.rst

@@ -2,7 +2,7 @@
 .. _box_tuple-bsize:
 
 ================================================================================
-box.tuple.bsize()
+tuple_object.bsize()
 ================================================================================
 
 .. class:: tuple_object

doc/reference/reference_lua/box_tuple/format.rst

@@ -0,0 +1,61 @@
+.. _box_tuple-format:
+
+tuple_object:format()
+=====================
+
+.. class:: tuple_object
+
+    .. method:: format()
+
+        Get the format of a tuple. The resulting table lists the fields of a tuple
+        (their names and types) if the ``format`` option was specified during the tuple
+        creation. Otherwise, the return value is empty.
+
+        :return: the tuple format.
+        :rtype:  table
+
+        .. note::
+
+            ``tuple_object.format()`` is equivalent to ``box.tuple.format(tuple_object)``.
+
+        Example:
+
+        *   A formatted tuple:
+
+            .. code-block:: tarantoolsession
+
+                tarantool> f = box.tuple.format.new({{'id', 'number'}, {'name', 'string'}})
+                ---
+                ...
+
+                tarantool> ftuple = box.tuple.new({1, 'Bob'}, {format = f})
+                ---
+                ...
+
+                tarantool> ftuple:format()
+                ---
+                - [{'name': 'id', 'type': 'number'}, {'name': 'name', 'type': 'string'}]
+                ...
+
+                tarantool> box.tuple.format(ftuple)
+                ---
+                - [{'name': 'id', 'type': 'number'}, {'name': 'name', 'type': 'string'}]
+                ...
+
+        *   A tuple without a format:
+
+            .. code-block:: tarantoolsession
+
+                tarantool> tuple1 = box.tuple.new({1, 'Bob'}) -- no format
+                ---
+                ...
+
+                tarantool> tuple1:format()
+                ---
+                - []
+                ...
+
+                tarantool> box.tuple.format(tuple1)
+                ---
+                - []
+                ...