git-fetch.txt: document fetch output

git-fetch.txt: document fetch output

This documents the ref update status of fetch. The structure of this
output is defined in [1]. The ouput content is refined a bit in [2]
[3] [4].

This patch is a copy from git-push.txt, modified a bit because the
flag '-' means different things in push (delete) and fetch (tag
update).

PS. For code archaeologists, the discussion mentioned in [1] is
probably [5].

[1] 165f390 (git-fetch: more terse fetch output - 2007-11-03)
[2] 6315472 (fetch: report local storage errors ... - 2008-06-26)
[3] f360d84 (builtin-fetch: add --prune option - 2009-11-10)
[4] 0997ada (fetch: describe new refs based on where... - 2012-04-16)
[5] http://thread.gmane.org/gmane.comp.version-control.git/61657

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>