R1:ac35246d0d54

Never sever non-cluster database; write more read-only documentation

Summary:
Ref T4571. Write more of the missing documentation sections and clarify a few things.

Since the "replicating master" check needs a special permission, imposes a performance penalty, is probably very difficult to misconfigure, and likely not a big deal anyway, just drop the idea of trying to automatically detect + prevent it. We still show if it's an issue on the status page, provided we have permission to check.

When you don't…