fetch doc: update introductory part for clarity
- "Branches" is a more common way to say "heads" in these days.
- Remote-tracking branches are used a lot more these days and it is
worth mentioning that it is one of the primary side effects of
the command to update them.
- Avoid "X. That means Y." If Y is easier to understand to
readers, just say that upfront.
- Use of explicit refspec to fetch tags does not have much to do
with turning "auto following" on or off. It is a way to fetch
tags that otherwise would not be fetched by auto-following.
Helped-by: Marc Branchaud <marcnarc@xiplink.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>