fetch doc: remove "short-cut" section

fetch doc: remove "short-cut" section

It is misleading to mention that <ref> that does not store is to
fetch the ref into FETCH_HEAD, because a refspec that does store is
also to fetch the LHS into FETCH_HEAD.  It is doubly misleading to
list it as part of "short-cut".  <ref> stands for a refspec that has
it on the LHS with a colon and an empty RHS, and that definition
should be given at the beginning of the entry where the format is
defined.

Tentatively remove this misleading description, which leaves the
`tag <tag>` as the only true short-hand, so move it at the beginning
of the entry.

Signed-off-by: Junio C Hamano <gitster@pobox.com>