Add description of box.error.is function

Function allows check whether the argument is an error. Added as a new page
Fixes #4236

Author: AArdeev

doc/reference/reference_lua/box_error.rst

@@ -275,6 +275,9 @@ Below is a list of ``box.error`` functions and related objects.
 
         *   - :doc:`./box_error/set`
             - Set the specified error as the last system error explicitly
+        
+        *   - :doc:`./box_error/is`
+            - Verify whether the specified argument is an error
 
         *   - :doc:`./box_error/error_object`
             - An object that defines an error
@@ -288,4 +291,5 @@ Below is a list of ``box.error`` functions and related objects.
     box_error/clear
     box_error/new
     box_error/set
+    box_error/is
     box_error/error_object

doc/reference/reference_lua/box_error/is.rst

@@ -0,0 +1,28 @@
+.. _box_error-is:
+
+===============================================================================
+box.error.is()
+===============================================================================
+
+.. function:: box.error.is(object_name)
+
+    **Since:** :doc:`3.2.0 </release/3.2.0>`
+
+    The ``box.error.is`` function allows verify whether the specified argument is an error.
+
+    :param object_name object_name: the subject of the request
+
+    **Return type:**
+    boolean
+
+    **Example**
+
+        ..  code-block:: lua
+            tarantool> box.error.is(box.error.new(box.error.UNKNOWN))
+            ---
+            - true
+            ...
+            tarantool> box.error.is('foo')
+            ---
+            - false
+            ...