trailer --only-input: prefer "configuration variables" over "rules"

trailer --only-input: prefer "configuration variables" over "rules"

Use the phrase "configuration variables" instead of "rules" because

(1) we already say "configuration variables" in multiple
    places in the docs (where the word "rules" is only used for describing
    "--only-input" behavior and for an unrelated case of mentioning how
    the trailers do not follow "rules for RFC 822 headers"), and

(2) this phrase is more specific than just "rules".

Signed-off-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>