bpo-43689: improve documentation for Differ (GH-25132)

Lines beginning with ``?`` try to help understanding the given diff.

The output can be hard to understand when it contains whitespace characters, such as spaces, tabs or line breaks.

While previously only tabs were mentioned, now all are listed.

Automerge-Triggered-By: GH:rhettinger

Doc/library/difflib.rst

@@ -79,7 +79,7 @@ diffs. For comparing directories and files, see also, the :mod:`filecmp` module.
 
    Lines beginning with '``?``' attempt to guide the eye to intraline differences,
    and were not present in either input sequence. These lines can be confusing if
-   the sequences contain tab characters.
+   the sequences contain whitespace characters, such as spaces, tabs or line breaks.
 
 
 .. class:: HtmlDiff

Misc/NEWS.d/next/Documentation/2021-04-01-08-09-34.bpo-43689.mqCfLe.rst

@@ -0,0 +1 @@
+The ``Differ`` documentation now also mentions other whitespace characters, which make it harder to understand the diff output.