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>

Documentation/email-clients.txt

@@ -211,7 +211,7 @@ Thunderbird (GUI)
 Thunderbird is an Outlook clone that likes to mangle text, but there are ways
 to coerce it into behaving.
 
-- Allows use of an external editor:
+- Allow use of an external editor:
   The easiest thing to do with Thunderbird and patches is to use an
   "external editor" extension and then just use your favorite $EDITOR
   for reading/merging patches into the body text.  To do this, download
@@ -219,6 +219,15 @@ to coerce it into behaving.
   View->Toolbars->Customize... and finally just click on it when in the
   Compose dialog.
 
+  Please note that "external editor" requires that your editor must not
+  fork, or in other words, the editor must not return before closing. 
+  You may have to pass additional flags or change the settings of your
+  editor. Most notably if you are using gvim then you must pass the -f
+  option to gvim by putting "/usr/bin/gvim -f" (if the binary is in
+  /usr/bin) to the text editor field in "external editor" settings. If you
+  are using some other editor then please read its manual to find out how
+  to do this.
+
 To beat some sense out of the internal editor, do this:
 
 - Edit your Thunderbird config settings so that it won't use format=flowed.