Manage and monitor Query Sequential Scan

Query without index  will be available by default for all deployments. However, for self managed and provisioned, we want to document:

1. How to turn it off with n1ql_feat_ctrl {"n1ql-feat-ctrl":"0x404c"}. We should use this opportunity to document how to change n1ql-feat-ctrl, ie.  change a setting and avoid overwriting previous setting. 
2. How to identify queries that uses sequential scan, i.e. query completed_requests where phaseTimes.primaryScan is not null.