docs: add and use include template for config/* includes

docs: add and use include template for config/* includes

In b6a8d09f6d8 (gc docs: include the "gc.*" section from "config" in
"gc", 2019-04-07) the "git gc" documentation was made to include the
config/gc.txt in its "CONFIGURATION" section. We do that in several
other places, but "git gc" was the only one with a blurb above the
include to orient the reader.

We don't want readers to carefully scrutinize "git-config(1)" and
"git-gc(1)" looking for discrepancies, instead we should tell them
that the latter includes a part of the former.

This change formalizes that wording in two new templates to be
included, one for the "git gc" case where the entire section is
included from "git-config(1)", and another for when the inclusion of
"git-config(1)" follows discussion unique to that documentation. In
order to use that re-arrange the order of those being discussed in the
"git-merge(1)" documentation.

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>