clarify when the list of subdirectories is read (closes #13779)
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 71133c4..f67ca25 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -1604,9 +1604,11 @@
If optional argument *topdown* is ``True`` or not specified, the triple for a
directory is generated before the triples for any of its subdirectories
- (directories are generated top-down). If *topdown* is ``False``, the triple for a
- directory is generated after the triples for all of its subdirectories
- (directories are generated bottom-up).
+ (directories are generated top-down). If *topdown* is ``False``, the triple
+ for a directory is generated after the triples for all of its subdirectories
+ (directories are generated bottom-up). No matter the value of *topdown*, the
+ list of subdirectories is retrieved before the tuples for the directory and
+ its subdirectories are generated.
When *topdown* is ``True``, the caller can modify the *dirnames* list in-place
(perhaps using :keyword:`del` or slice assignment), and :func:`walk` will only