Document force_union_syntax and force_uppercase_builtins (#16048)

Users don't know about them:
typeddjango/pytest-mypy-plugins#126
Since they are quite important for testing, I think that it is a must to
include them.

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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

docs/source/config_file.rst

@@ -816,6 +816,22 @@ These options may only be set in the global section (``[mypy]``).
 
     Show absolute paths to files.
 
+.. confval:: force_uppercase_builtins
+
+    :type: boolean
+    :default: False
+
+    Always use ``List`` instead of ``list`` in error messages,
+    even on Python 3.9+.
+
+.. confval:: force_union_syntax
+
+    :type: boolean
+    :default: False
+
+    Always use ``Union[]`` and ``Optional[]`` for union types
+    in error messages (instead of the ``|`` operator),
+    even on Python 3.10+.
 
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