git-config: update doc for --get with multiple values

Since commit 00b347d (git-config: do not complain about duplicate
entries, 2012-10-23), "git config --get" does not exit with an error if
there are multiple values for the specified key but instead returns the
last value.  Update the documentation to reflect this.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>