[Orleans] Update Reminders docs to explicitly indicate that heterogenous clusters must be configured identically

### Describe the issue or suggestion

When running in a heterogenous setup (where different hosts have different sets of supported grains), developers must be sure to keep their Reminders configuration consistent.

That is, all hosts in a cluster must have the same configuration for reminders.
This is also true for streams, so those docs should also be updated.
@benjaminpetit can you think of other places where we should be clear about this?