Issue #27180: Clarify Path.rename() behavior on Unix systems

Patch by Evelyn Mitchell.

Doc/library/pathlib.rst

@@ -891,8 +891,9 @@ call fails (for example because the path doesn't exist):
 
 .. method:: Path.rename(target)
 
-   Rename this file or directory to the given *target*.  *target* can be
-   either a string or another path object::
+   Rename this file or directory to the given *target*.  On Unix, if
+   *target* exists and is a file, it will be replaced silently if the user
+   has permission.  *target* can be either a string or another path object::
 
       >>> p = Path('foo')
       >>> p.open('w').write('some text')