pythongh-111264: Add a note about untrusted input to tomllib docs (pythonGH-146209)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: encukou

Doc/library/tomllib.rst

@@ -19,6 +19,12 @@ support writing TOML.
    Added TOML 1.1.0 support.
    See the :ref:`What's New <whatsnew315-tomllib-1-1-0>` for details.
 
+.. warning::
+
+   Be cautious when parsing data from untrusted sources.
+   A malicious TOML string may cause the decoder to consume considerable
+   CPU and memory resources.
+   Limiting the size of data to be parsed is recommended.
 
 .. seealso::