[3.13] gh-111264: Add a note about untrusted input to tomllib docs (#149226)

(cherry picked from commit 9d41e2a534)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Stan Ulbrych <stan@python.org>

Doc/library/tomllib.rst

@@ -17,6 +17,13 @@ This module provides an interface for parsing TOML 1.0.0 (Tom's Obvious Minimal
 Language, `https://toml.io <https://toml.io/en/>`_). This module does not
 support writing TOML.
 
+.. 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::
 
     The :pypi:`Tomli-W package <tomli-w>`