Drop the description of "plain-text" format

Change-Id: Ib1098ce4eb50bd6453ef51e4307ebe1c75e75ea6
Reviewed-on: http://review.couchbase.org/27809
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>