rustdoc: summary line for hg_path_to_os_string
The main motivation of this change is to avoid the TODO being
the summary line, even though this leads to a pretty obvious summary.
Then doc-comments for the other functions are introduced for
consistency.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}