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 e0e34e977a7c ("Documentation/email-clients.txt:
update Thunderbird docs with wordwrap plugin") but then removed in commit
f9a0974d3f70 ("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>