docs: add CONFIGURATION sections that map to a built-in

docs: add CONFIGURATION sections that map to a built-in

Add a CONFIGURATION section to the documentation of various built-ins,
for those cases where the relevant config/NAME.txt describes
configuration that is only used by the relevant built-in documented in
git-NAME.txt. Subsequent commits will handle more complex cases.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>