debugdiscovery: correct and clean up command synopsis
This does a few things:
* Changes "-r" to "--rev", since "-r" is not a valid short form
* Removes non-existent "-l" and "-b" options
* Removes "..." after options, since we don't usually have that
Differential Revision: https://phab.mercurial-scm.org/D1706
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"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}