git-config.1: --get-all description update

git-config.1: --get-all description update

"git config --get-all foo.bar" shows all values for the foo.bar
variable, but does not give the variable name in each output entry.
Hence it is equivalent to "git config get --all foo.bar", without
"--show-names", in the more modern syntax.

Signed-off-by: Junio C Hamano <gitster@pobox.com>