repository: clarify role of imanifestlog
Looking at the implementation of manifest.manifestlog, it is highly
tailored towards servicing the root manifest. So clarify that in the
interface docstring.
Differential Revision: https://phab.mercurial-scm.org/D4273
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}