Reword "detached HEAD" notification

The old "advice" message explained how to create a branch after going into
a detached HEAD state but didn't make it clear why the user may want to do
so.  Also "moving to ... which isn't a local branch" was unclear if it is
complaining, if it is describing the new state, or if it is explaining why
the HEAD is detached (the true reason is the last one).

Give the established phrase 'detached HEAD' first to make it easy for
users to look up the concept in documentation, and briefly describe what
can be done in the state (i.e. play around without having to clean up)
before telling the user how to keep what was done during the temporary
state.

Allow the long description to be hidden by setting advice.detachedHead
configuration to false.

We might want to customize the advice depending on how the commit to check
out was spelled (e.g. instead of "new-branch-name", we way want to say
"topic" when "git checkout origin/topic" triggered this message) in later
updates, but this encapsulates that into a separate function and it should
be a good first step.

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

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