doc: git-checkout: clarify -b and -B

From user feedback: several users are having trouble understanding the
difference between `-b` and `-B` ("I think it's because my brain expects
it to contrast with `-b`, but instead it starts off explaining how
they're the same").

Also, in `-B`, 2 users can't tell what the branch is reset *to*.

To fix all of these, split `-B` out and describe `-B` as "it's the same
as `-b`, except...". Move all of the other contents of `-B` to `-b`.

Update the example of why `git checkout -b` might fail so that it's
appropriate for `-b`, explain what `<start-point>` for `-b` means,
and (now that we've added extra information to `-b`) move some of
unnecessary detail about `--track` down to the OPTIONS section.

Signed-off-by: Julia Evans <[email protected]>

Author: jvns

Documentation/git-checkout.adoc

@@ -46,27 +46,21 @@ $ git checkout -b <branch> --track <remote>/<branch>
 Running `git checkout` without specifying a branch has no effect except
 to print out the tracking information for the current branch.
 
-`git checkout (-b|-B) <new-branch> [<start-point>]`::
-
-	Specifying `-b` causes a new branch to be created as if
-	linkgit:git-branch[1] were called and then checked out.  In
-	this case you can use the `--track` or `--no-track` options,
-	which will be passed to `git branch`.  As a convenience,
-	`--track` without `-b` implies branch creation; see the
-	description of `--track` below.
-+
-If `-B` is given, _<new-branch>_ is created if it doesn't exist; otherwise, it
-is reset. This is the transactional equivalent of
-+
-------------
-$ git branch -f <branch> [<start-point>]
-$ git checkout <branch>
-------------
+`git checkout -b <new-branch> [<start-point>]`::
+
+	Create a new branch named _<new-branch>_, start it at _<start-point>_
+	(defaults to the current commit), and check out the new branch.
+	You can use the `--track` or `--no-track` options to set the branch's
+	upstream tracking information.
 +
-that is to say, the branch is not reset/created unless "git checkout" is
-successful (e.g., when the branch is in use in another worktree, not
-just the current branch stays the same, but the branch is not reset to
-the start-point, either).
+This fails without making any changes if there's an error checking out
+_<new-branch>_, for example if checking out the `<start-point>`
+commit would overwrite your uncommitted changes.
+
+`git checkout -B <branch> [<start-point>]`::
+
+	The same as `-b`, except that if the branch already exists it
+	resets `_<branch>_` to the start point instead of failing.
 
 `git checkout --detach [<branch>]`::
 `git checkout [--detach] <commit>`::
@@ -156,16 +150,14 @@ of it").
 	see linkgit:git-branch[1] for details.
 
 `-B <new-branch>`::
-	Creates the branch _<new-branch>_, start it at _<start-point>_;
-	if it already exists, then reset it to _<start-point>_. And then
-	check the resulting branch out.  This is equivalent to running
-	`git branch` with `-f` followed by `git checkout` of that branch;
-	see linkgit:git-branch[1] for details.
+	The same as `-b`, except that if the branch already exists it
+	resets `_<branch>_` to the start point instead of failing.
 
 `-t`::
 `--track[=(direct|inherit)]`::
 	When creating a new branch, set up "upstream" configuration. See
-	`--track` in linkgit:git-branch[1] for details.
+	`--track` in linkgit:git-branch[1] for details. As a convenience,
+	--track without -b implies branch creation.
 +
 If no `-b` option is given, the name of the new branch will be
 derived from the remote-tracking branch, by looking at the local part of