docs/config: give a relative includeIf example

docs/config: give a relative includeIf example

The changes in the previous commit hopefully clarify that
the evaluation of an include "path" variable is the same no
matter if it's in a conditional section or not. But since
this question came up on the list, let's add an example that
makes it obvious.

Signed-off-by: Jeff King <redacted>
Reviewed-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>