docs: process: email-client: add Thunderbird "Toggle Line Wrap" extension

While reading the git-format-patch manpages [1], I discovered the existence
of the "Toggle Line Wrap" extension for Thunderbird which I found rather
convenient.

Looking at the history, the ancestor of this extension was added to the
documentation in commit e0e34e977a ("Documentation/email-clients.txt:
update Thunderbird docs with wordwrap plugin") but then removed in commit
f9a0974d3f ("Documentation: update thunderbird email client settings").

Extend the paragraph on Thunderbird's mailnews.wraplength register to
mention the existence of the "Toggle Line Wrap" extension. The goal is not
to create a war on what is the best option so make it clear that this is
just an alternative.

[1] man git-format-patch -- §Thunderbird
Link: https://git-scm.com/docs/git-format-patch#_thunderbird

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Acked-by: Sotir Danailov <sndanailov@gmail.com>  # As past commit author
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251226-docs_thunderbird-toggle-line-wrap-v2-1-aebb8c60025d@kernel.org>

Documentation/process/email-clients.rst

@@ -324,7 +324,14 @@ To beat some sense out of the internal editor, do this:
 
   - Set ``mailnews.send_plaintext_flowed`` to ``false``
 
-  - Set ``mailnews.wraplength`` from ``72`` to ``0``
+  - Set ``mailnews.wraplength`` from ``72`` to ``0`` **or** install the
+    "Toggle Line Wrap" extension
+
+    https://github.com/jan-kiszka/togglelinewrap
+
+    https://addons.thunderbird.net/thunderbird/addon/toggle-line-wrap
+
+    to control this registry on the fly.
 
 - Don't write HTML messages! Go to the main window
   :menuselection:`Main Menu-->Account Settings-->youracc@server.something-->Composition & Addressing`!