Documentation: update thunderbird email client settings

Added setting to email-clients that is easier to read and is easier to
setup thunderbird.  Removed config settings and added GUI settings.

Signed-off-by: Paul McQuade <paulmcquad@gmail.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>