Documentation/email-clients.txt: update gmail information

Gmail web gui does not work for sending patches now even with firefox
"view source with" extension.  It will use windows style line breaks to
wrap lines automatically when sening email.

Rewrite the gmail web gui part of email client documentation.

Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Cc: Martin Bligh <mbligh@mbligh.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>