docs/config: avoid the term "expand" for includes

docs/config: avoid the term "expand" for includes

Using the word "expand" to refer to including the contents
of another config file isn't really accurate, since it's a
verbatim insertion. And it can cause confusion with the
expanding of the path itself via things like "~".

Let's clarify when we are referring to the contents versus
the filename, and use appropriate verbs in each case.

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>