fetch doc: add a section on configured remote-tracking branches

To resurrect a misleading mention removed in the previous step,
add a section to explain how the remote-tracking configuration
interacts with the refspecs given as the command-line arguments.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
fetch: allow explicit --refmap to override configuration

Since the introduction of opportunisitic updates of remote-tracking
branches, started at around f2690487 (fetch: opportunistically
update tracking refs, 2013-05-11) with a few updates in v1.8.4 era,
the remote.*.fetch configuration always kicks in even when a refspec
to specify what to fetch is given on the command line, and there is
no way to disable or override it per-invocation.

Teach the command to pay attention to the --refmap=<lhs>:<rhs>
command-line options that can be used to override the use of
configured remote.*.fetch as the refmap.

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