As described in the commit message of 9b25a0b (config: add
include directive, 2012-02-06), the `--include` option is
only on by default in some cases. But our documentation
described it as just "defaults to on", which doesn't tell
the whole story.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>