Improve docs for serviceAccount/RBAC-related settings

[gerlowskija]

Users can customize the RBAC permissions given to the operator by various "serviceAccount" helm chart settings. But the docs for these settings don't go into much detail on what permissions are given by default, what permissions are "optional", etc.

Additionally, some other settings (e.g. solrprometheusexporter's .Spec.SolrReference.Cloud.Namespace) deserve a special call out for their RBAC implications.