Document force_union_syntax and force_lowercase_builtins

Author: sobolevn

docs/source/config_file.rst

@@ -816,6 +816,19 @@ These options may only be set in the global section (``[mypy]``).
 
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+.. confval:: force_lowercase_builtins
+
+    :type: boolean
+    :default: False
+
+    Force to use ``List`` instead of ``list`` .
+
+.. confval:: force_union_syntax
+
+    :type: boolean
+    :default: True
+
+    Force to use ``Union[]`` type instead of ``|`` operator.
 
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