GH-103475: cache() and lru_cache() do not have a "call once" guarantee

Doc/library/functools.rst

@@ -49,8 +49,13 @@ The :mod:`functools` module defines the following functions:
         >>> factorial(12)      # makes two new recursive calls, the other 10 are cached
         479001600
 
-   The cache is threadsafe so the wrapped function can be used in multiple
-   threads.
+   The cache is threadsafe so that the wrapped function can be used in
+   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.
 
    .. versionadded:: 3.9
 
@@ -159,8 +164,13 @@ The :mod:`functools` module defines the following functions:
    *maxsize* most recent calls.  It can save time when an expensive or I/O bound
    function is periodically called with the same arguments.
 
-   The cache is threadsafe so the wrapped function can be used in multiple
-   threads.
+   The cache is threadsafe so that the wrapped function can be used in
+   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.
 
    Since a dictionary is used to cache results, the positional and keyword
    arguments to the function must be :term:`hashable`.