update shutil.which doc

Author: 985025074

Doc/library/shutil.rst

@@ -445,6 +445,8 @@ Directory and files operations
 
    Return the path to an executable which would be run if the given *cmd* was
    called.  If no *cmd* would be called, return ``None``.
+   If *cmd* contains a directory component, *which* will only check the specified path directly
+   and will not search the directories listed in *path* or in the system's ``PATH`` environment variable.
 
    *mode* is a permission mask passed to :func:`os.access`, by default
    determining if the file exists and is executable.