docs: infrastructure roles startup, secret namespace and per-cluster … - #3225

Problem description

Several infrastructure role behaviours are only visible in the code, which leads to roles that are silently not created (#1764, #1618):

• the role definitions are read when the operator starts: a secret created or changed afterwards is ignored until the next restart
• a secretname without namespace refers to the operator namespace with the CRD configuration, but to default with the ConfigMap
• defaultuservalue / defaultrolevalue are not documented
• the per-cluster credentials secret the operator creates for each infrastructure role (in the cluster namespace, kept in sync with password changes) is not documented, although it is the natural way for sidecars or exporters to get the credentials

Changes

• docs/user.md: short "Good to know" list in the infrastructure roles section, with a postgres_exporter / pg_monitor example
• docs/reference/operator_parameters.md: ConfigMap format example for infrastructure_roles_secrets and a link to the section

Refs #1764, #1618