Update man pages regarding IO plugins

Change-Id: Iec67222ffd998c4cca4d106390265aed5aa13d19
Reviewed-on: http://review.couchbase.org/28447
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>