No more than 1 instance of Che/CRW per cluster supported

Make sure docs contain a warning that only 1 instance of Che/CRW on a cluster is supported.

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As discussed in CRW-2367 and noted in https://access.redhat.com/documentation/en-us/red_hat_codeready_workspaces/2.11/html-single/installation_guide/index#installing-codeready-workspaces-on-openshift-3-using-the-operator_crw we have some mention(s) in the CRW install docs about multiple instances on the same cluster.

This should be removed immediately and replaced by installation guide note stating that multiple instances in the same cluster is NOT SUPPORTED AT ALL, as this approach is doing to be completely impossible once 2.14 is live.