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