Documentation/email-clients.txt: Fix one grammar mistake, add extra info about TB

Fix one grammar mistake (Allows->Allow) and add extra information about
the external editor add-on of Thunderbird: the developer must make sure
that their editor doesn't fork (IOW it mustn't return before closing)
thus they should be careful how they configure the addon. Furthermore,
add a tip how to do it with gvim.

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@gmail.com>
[jc: some minor wording/formatting tweaks]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>