revlog: add some comment in the header sections
We are about to add more content so let us organise the existing content first.
Differential Revision: https://phab.mercurial-scm.org/D10302
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}