docs/config: clarify include/includeIf relationship

docs/config: clarify include/includeIf relationship

The "includeIf" directives behave exactly like include ones,
except they only kick in when the conditional is true. That
was mentioned in the "conditional" section, but let's make
it more clear for the whole "includes" section, since people
don't necessarily read the documentation top to bottom.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>