doc: add description about location of management files for subrepo

doc: add description about re-opening closed heads to 'Head, closed branch' in glossary

doc: fix careless document miss for term 'Close changeset' in glossary there is no term 'Changeset, close' in glossary. 'Close changeset' seems to have to be linked not to 'Branch, closed', but to 'Head, closed branch', because only the latter explains about "the changeset that marks a head as no longer interesting".