githelp: add some minimal help for pickaxe functionality
Due to a conversation in work chat, I realized this is actually pretty
well-hidden in Mercurial.
Differential Revision: https://phab.mercurial-scm.org/D8590
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}