Fix manpages for HTTP callbacks

Change-Id: I6017d7569b610a08f3342a2b3db6572fea5bcc45
Reviewed-on: http://review.couchbase.org/25385
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>