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>

Documentation/git-checkout.txt

@@ -9,7 +9,8 @@ SYNOPSIS
 --------
 [verse]
 'git checkout' [-q] [-f] [-m] [<branch>]
-'git checkout' [-q] [-f] [-m] [--detach] [<commit>]
+'git checkout' [-q] [-f] [-m] --detach [<branch>]
+'git checkout' [-q] [-f] [-m] [--detach] <commit>
 'git checkout' [-q] [-f] [-m] [[-b|-B|--orphan] <new_branch>] [<start_point>]
 'git checkout' [-f|--ours|--theirs|-m|--conflict=<style>] [<tree-ish>] [--] <paths>...
 'git checkout' [-p|--patch] [<tree-ish>] [--] [<paths>...]
@@ -62,7 +63,7 @@ that is to say, the branch is not reset/created unless "git checkout" is
 successful.
 
 'git checkout' --detach [<branch>]::
-'git checkout' <commit>::
+'git checkout' [--detach] <commit>::
 
 	Prepare to work on top of <commit>, by detaching HEAD at it
 	(see "DETACHED HEAD" section), and updating the index and the
@@ -71,10 +72,11 @@ successful.
 	tree will be the state recorded in the commit plus the local
 	modifications.
 +
-Passing `--detach` forces this behavior in the case of a <branch> (without
-the option, giving a branch name to the command would check out the branch,
-instead of detaching HEAD at it), or the current commit,
-if no <branch> is specified.
+When the <commit> argument is a branch name, the `--detach` option can
+be used to detach HEAD at the tip of the branch (`git checkout
+<branch>` would check out that branch without detaching HEAD).
++
+Omitting <branch> detaches HEAD at the tip of the current branch.
 
 'git checkout' [-p|--patch] [<tree-ish>] [--] <pathspec>...::