format-patch doc: Thunderbird wraps lines unless mailnews.wraplength=0

The Thunderbird section of the 'MUA-specific hints' contains three
different approaches to setting up the mail client to leave patch
emails unmolested. The second approach (configuration) has a step
missing when configuring the composition window not to wrap. In
particular, the "mailnews.wraplength" configuration variable needs
to be set to zero. Update the documentation to add the missing
setting.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

Documentation/git-format-patch.txt

@@ -438,7 +438,8 @@ Edit..Preferences..Composition, wrap plain text messages at 0
 In Thunderbird 3:
 Edit..Preferences..Advanced..Config Editor.  Search for
 "mail.wrap_long_lines".
-Toggle it to make sure it is set to `false`.
+Toggle it to make sure it is set to `false`. Also, search for
+"mailnews.wraplength" and set the value to 0.
 
 3. Disable the use of format=flowed:
 Edit..Preferences..Advanced..Config Editor.  Search for