Documentation: email-clients: Add better Thunderbird information

Add better Thunderbird information.
Add Thunderbird Registry instructions to:
  Enable UTF8 & Preformat mode
  Disable HTML mode

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