#6647: document that catch_warnings is not thread-safe
diff --git a/Doc/library/warnings.rst b/Doc/library/warnings.rst
index e4343e8..91ab085 100644
--- a/Doc/library/warnings.rst
+++ b/Doc/library/warnings.rst
@@ -383,6 +383,14 @@
.. note::
+ The :class:`catch_warnings` manager works by replacing and
+ then later restoring the module's
+ :func:`showwarning` function and internal list of filter
+ specifications. This means the context manager is modifying
+ global state and therefore is not thread-safe.
+
+ .. note::
+
In Python 3.0, the arguments to the constructor for
:class:`catch_warnings` are keyword-only arguments.