doc: advertise GIT_CONFIG_NOSYSTEM

doc: advertise GIT_CONFIG_NOSYSTEM

On a multiuser system where mortals do not have write access to /etc,
the GIT_CONFIG_NOSYSTEM variable is the best tool we have to keep
getting work done when a syntax error or other problem renders
/etc/gitconfig buggy, until the sysadmin sorts the problem out.

Noticed while experimenting with teaching git to error out when
/etc/gitconfig is unreadable.

Signed-off-by: Jonathan Nieder <redacted>
Acked-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>