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>

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