@@ -655,9 +655,6 @@ function.
    Accepts a wide range of Python callables, from plain functions and classes to
    :func:`functools.partial` objects.
 
-   If the passed object has a ``__signature__`` attribute, this function
-   returns it without further computations.
-
    For objects defined in modules using stringized annotations
    (``from __future__ import annotations``), :func:`signature` will
    attempt to automatically un-stringize the annotations using
@@ -692,6 +689,13 @@ function.
       Python.  For example, in CPython, some built-in functions defined in
       C provide no metadata about their arguments.
 
+   .. impl-detail::
+
+      If the passed object has a :attr:`!__signature__` attribute,
+      we may use it to create the signature.
+      The exact semantics are an implementation detail and are subject to
+      unannounced changes. Consult the source code for current semantics.
+
 
 .. class:: Signature(parameters=None, *, return_annotation=Signature.empty)