Fix over-simplified documentation for 'git log -z'

Fix over-simplified documentation for 'git log -z'

In commit 64485b4a, the documentation for 'git log -z' was
simplified too much. The -z option actually changes the behavior
of 'git log' in two ways: commits will be ended with a NUL
instead of a LF (correctly documented) and the --raw and
--numstat will have NUL as field terminators (omitted in
the documentation for 'git log').

Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>