Documentation: detached HEAD

Add discussion section to git-checkout documentation and mention
detached HEAD in repository-layout document.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-checkout.txt

@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git-checkout' [-f] [-b <new_branch> [-l]] [-m] [<branch>]
-'git-checkout' [-m] [<branch>] <paths>...
+'git-checkout' [<branch>] <paths>...
 
 DESCRIPTION
 -----------
@@ -64,6 +64,56 @@ and mark the resolved paths with `git update-index`.
 <branch>::
 	Branch to checkout; may be any object ID that resolves to a
 	commit.  Defaults to HEAD.
++
+When this parameter names a non-branch (but still a valid commit object),
+your HEAD becomes 'detached'.
+
+
+Detached HEAD
+-------------
+
+It is sometimes useful to be able to 'checkout' a commit that is
+not at the tip of one of your branches.  The most obvious
+example is to check out the commit at a tagged official release
+point, like this:
+
+------------
+$ git checkout v2.6.18
+------------
+
+Earlier versions of git did not allow this and asked you to
+create a temporary branch using `-b` option, but starting from
+version 1.5.0, the above command 'detaches' your HEAD from the
+current branch and directly point at the commit named by the tag
+(`v2.6.18` in the above example).
+
+You can use usual git commands while in this state.  You can use
+`git-reset --hard $othercommit` to further move around, for
+example.  You can make changes and create a new commit on top of
+a detached HEAD.  You can even create a merge by using `git
+merge $othercommit`.
+
+The state you are in while your HEAD is detached is not recorded
+by any branch (which is natural --- you are not on any branch).
+What this means is that you can discard your temporary commits
+and merges by switching back to an existing branch (e.g. `git
+checkout master`), and a later `git prune` or `git gc` would
+garbage-collect them.
+
+The command would refuse to switch back to make sure that you do
+not discard your temporary state by mistake when your detached
+HEAD is not pointed at by any existing ref.  If you did want to
+save your state (e.g. "I was interested in the fifth commit from
+the top of 'master' branch", or "I made two commits to fix minor
+bugs while on a detached HEAD" -- and if you do not want to lose
+these facts), you can create a new branch and switch to it with
+`git checkout -b newbranch` so that you can keep building on
+that state, or tag it first so that you can come back to it
+later and switch to the branch you wanted to switch to with `git
+tag that_state; git checkout master`.  On the other hand, if you
+did want to discard the temporary state, you can give `-f`
+option (e.g. `git checkout -f master`) to override this
+behaviour.
 
 
 EXAMPLES

Documentation/repository-layout.txt

@@ -91,6 +91,12 @@ HEAD::
 	'name' does not (yet) exist.  In some legacy setups, it is
 	a symbolic link instead of a symref that points at the current
 	branch.
++
+HEAD can also record a specific commit directly, instead of
+being a symref to point at the current branch.  Such a state
+is often called 'detached HEAD', and almost all commands work
+identically as normal.  See gitlink:git-checkout[1] for
+details.
 
 branches::
 	A slightly deprecated way to store shorthands to be used