Clarify in the documentation the recommendation of setting `dispatchinterval` to 0 applies to non-admin nodes only

As per a conversation with @mostolog in the mailing list, the following snippet in the documentation does not seem to be 100% clear:

To ensure that jobs are not dispatched by non-admin nodes you may also want to set:

dispatchinterval=0

It could be easily interpreted as "the setting may be applied to all nodes" instead of "the setting may be applied to all nodes, except the admin".