Document any TYPE_CHECKING name works (python#18443)

Author: hauntsaninja

docs/source/common_issues.rst

@@ -455,7 +455,7 @@ Example:
    # The rest of this file doesn't apply to Windows.
 
 Some other expressions exhibit similar behavior; in particular,
-:py:data:`~typing.TYPE_CHECKING`, variables named ``MYPY``, and any variable
+:py:data:`~typing.TYPE_CHECKING`, variables named ``MYPY`` or ``TYPE_CHECKING``, and any variable
 whose name is passed to :option:`--always-true <mypy --always-true>` or :option:`--always-false <mypy --always-false>`.
 (However, ``True`` and ``False`` are not treated specially!)