bpo-42958: Improve description of shallow= in filecmp.cmp docs (GH-27166)


Co-authored-by: Ɓukasz Langa <lukasz@langa.pl>
Co-authored-by: Alexander Vandenbulcke <alexander.vandenbulcke95@gmail.com>
(cherry picked from commit a8dc4893d2b28827e82447326ea47759c161a722)

Co-authored-by: andrei kulakov <andrei.avk@gmail.com>
diff --git a/Doc/library/filecmp.rst b/Doc/library/filecmp.rst
index c60603b..83e9e14 100644
--- a/Doc/library/filecmp.rst
+++ b/Doc/library/filecmp.rst
@@ -22,8 +22,11 @@
    Compare the files named *f1* and *f2*, returning ``True`` if they seem equal,
    ``False`` otherwise.
 
-   If *shallow* is true, files with identical :func:`os.stat` signatures are
-   taken to be equal.  Otherwise, the contents of the files are compared.
+   If *shallow* is true and the :func:`os.stat` signatures (file type, size, and
+   modification time) of both files are identical, the files are taken to be
+   equal.
+
+   Otherwise, the files are treated as different if their sizes or contents differ.
 
    Note that no external programs are called from this function, giving it
    portability and efficiency.