gh-131544: Update docs for PyType_AddWatcher (gh-132015)

diff --git a/Doc/c-api/type.rst b/Doc/c-api/type.rst
index 862f1e67b9ad2d526803f0a60328200ba898214f..ec2867b0ce09ba113e88dd027df3daa4fdc5ce0f 100644 (file)
--- a/Doc/c-api/type.rst
+++ b/Doc/c-api/type.rst
@@ -82,6 +82,9 @@ Type Objects
    error (e.g. no more watcher IDs available), return ``-1`` and set an
    exception.
 
+   In free-threaded builds, :c:func:`PyType_AddWatcher` is not thread-safe,
+   so it must be called at start up (before spawning the first thread).
+
    .. versionadded:: 3.12