dirstate-item: update the attribute documentation
It was very outdated. We are about to change these attribute so we should has
well have them documented so that the change get easier to grasp.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}