Add docs about --force-uppercase-builtins and --force-union-syntax (#16049)

Refs #16048

Author: sobolevn

docs/source/command_line.rst

@@ -789,6 +789,17 @@ in error messages.
     useful or they may be overly noisy. If ``N`` is negative, there is
     no limit. The default limit is 200.
 
+.. option:: --force-uppercase-builtins
+
+    Always use ``List`` instead of ``list`` in error messages,
+    even on Python 3.9+.
+
+.. option:: --force-union-syntax
+
+    Always use ``Union[]`` and ``Optional[]`` for union types
+    in error messages (instead of the ``|`` operator),
+    even on Python 3.10+.
+
 
 .. _incremental: