checkout: update synopsys and documentation on detaching HEAD

In the synopsis, the second form to detach HEAD at the named commit
labelled the argument as '<commit>'.  While this is technically more
correct, because the feature to detach is not limited to the tip of
a named branch, it was found confusing and did not express the fact
that you have to give `--detach` if you are naming the commit you
want to detach HEAD at with a branch name.

Separate this case into two syntactical forms, mimicking the way how
the DESCRIPTION section shows this usage.  Also update the text that
explains the syntax to name the commit to detach HEAD at to clarify.

Suggested-by: Benjamin Bergman <ben@benbergman.ca>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>