fetch doc: add a section on configured remote-tracking branches
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 <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git