user-manual: use 'git config --global user.*' for setup

user-manual: use 'git config --global user.*' for setup

A simple command line call is easier than spawning an editor,
especially for folks new to ideas like the "command line" and "text
editors".  This is also the approach suggested by 'git commit' if you
try and commit without having configured user.name or user.email.

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>