clonebundles: document REQUIREDRAM key
This is a follow-up to 72feaeb510b3, which introduced the feature. The
key should be documented as part of the format specification inside the
extension docstring.
Differential Revision: https://phab.mercurial-scm.org/D8838
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}