Documentation/glossary: define root refs as refs

Documentation/glossary: define root refs as refs

Except for the pseudorefs MERGE_HEAD and FETCH_HEAD, all refs that live
in the root of the ref hierarchy behave the exact same as normal refs.
They can be symbolic refs or direct refs and can be read, iterated over
and written via normal tooling. All of these refs are stored in the ref
backends, which further demonstrates that they are just normal refs.

Extend the definition of "ref" to also cover such root refs. The only
additional restriction for root refs is that they must conform to a
specific naming schema.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>