advice: Document that they all default to true

advice: Document that they all default to true

By definition, the default value of "advice.*" variables must be true and
they all control various additional help messages that are designed to aid
new users. Setting one to false is to tell Git that the user understands
the nature of the error and does not need the additional verbose help
message.

Also fix the asciidoc markup for linkgit:git-checkout[1] in the
description of the detachedHead advice by removing an excess colon.

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