git-clean: fix the description of the default behavior

git-clean: fix the description of the default behavior

Currently, when called without -n and -f, git clean issues

fatal: clean.requireForce not set and -n or -f not given; refusing to clean

which leaves the user wondering why force is required when requireForce
is not set. Looking up in git-clean(1) does not help because its
description is wrong.

Change it so that git clean issues

fatal: clean.requireForce defaults to true and -n or -f not given; refusing to clean

in this situation (and "...set to true..." when it is set) which makes
it clearer that an unset config means true here, and adjust the
documentation.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>