MB-10086: added api docs for clusterwide collectinfo info feature

Change-Id: Ic377ecfddee33cdaa42a6aa72d3ec99577f09449
Reviewed-on: http://review.couchbase.org/37826
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>