merge: document return values of manifestmerge() and calculateupdates()
In future patches, I want to add one more return value which represents
information which needs to stored and used at commit time.
Differential Revision: https://phab.mercurial-scm.org/D8741
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}