gh-141831:Document behavior of functools.lru_cache with concurrent access

Doc/library/functools.rst

@@ -50,13 +50,16 @@ The :mod:`functools` module defines the following functions:
         479001600
 
    The cache is threadsafe so that the wrapped function can be used in
-   multiple threads.  This means that the underlying data structure will
+   multiple threads. This means that the underlying data structure will
    remain coherent during concurrent updates.
 
    It is possible for the wrapped function to be called more than once if
    another thread makes an additional call before the initial call has been
    completed and cached.
 
+   Call-once behavior is not guaranteed: no locks are held during the function
+   call, so the same key may be computed multiple times concurrently.
+
    .. versionadded:: 3.9