checkout: advice how to get out of detached HEAD mode

checkout: advice how to get out of detached HEAD mode

Detached HEAD mode is considered dangerous and confusing for newcomers
and we print a big block of warning how to move forward. But we should
also suggest the user the way to get out of it if they get into detached
HEAD by mistake.

While at there, I also suggest how to turn the advice off. This is
another thing I find annoying with advices and should be dealt with in a
more generic way. But that may require some refactoring in advice.c
first.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>