persistent-nodemap: document the feature in `hg help config.format`
This is necessary before we advertise the feature more.
Differential Revision: https://phab.mercurial-scm.org/D9755
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}