Describe exptime argument in lcb_touch(3) manpage

Source: http://www.couchbase.com/communities/q-and-a/omission-lcbtouch-man-page
Change-Id: Ie74a05da1ab7cf160d3feb6b3b88ab1ad4546f64
Reviewed-on: http://review.couchbase.org/27718
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>