DOC: halog: explain how to use -ac and -ad in the help message

Tim reported in issue #1435 that halog options -ac/-ad were poorly
documented. They're indeed used to spot infrastructure outages between
the clients and haproxy by detecting abnormal periods of silence followed
by bursts, either affecting the network itself, or also a single machine
(e.g. swapping on an edge client or proxy can cause such patterns).
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