doc: git-checkout: clarify git checkout <branch>

- "To prepare for working on _<branch>_..." is confusing to some users:
  it makes it sound like these are steps that the user has to do, not
  steps that Git itself will do. Reword it.
- Use "changes" instead of "modifications" (which Git normally does)
- Mention that `git checkout` will fail if there's a merge conflict
- The current explanation of `You could omit <branch>`... is confusing
  to users (what are the "expensive side effects"? what's a better way
  of getting the same info?). Be more direct and mention that `git
  status` is likely a better option.

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

Author: jvns

Documentation/git-checkout.adoc

@@ -30,11 +30,12 @@ See ARGUMENT DISAMBIGUATION below for how Git decides which one to do.
 Here's a description of all of the modes:
 
 `git checkout [<branch>]`::
-	To prepare for working on _<branch>_, switch to it by updating
-	the index and the files in the working tree, and by pointing
-	`HEAD` at the branch. Local modifications to the files in the
-	working tree are kept, so that they can be committed to the
-	_<branch>_.
+	Switch to _<branch>_. This will update the files in the working tree,
+	point `HEAD` at the branch, and update the index. Local changes to
+	the files in the working tree are kept, so that they can be committed
+	to the _<branch>_. If the local changes conflict with the changes on
+	_<branch>_, no changes will be made and the checkout operation will
+	fail.
 +
 If _<branch>_ is not found but there does exist a tracking branch in
 exactly one remote (call it _<remote>_) with a matching name and
@@ -44,10 +45,9 @@ exactly one remote (call it _<remote>_) with a matching name and
 $ git checkout -b <branch> --track <remote>/<branch>
 ------------
 +
-You could omit _<branch>_, in which case the command degenerates to
-"check out the current branch", which is a glorified no-op with
-rather expensive side-effects to show only the tracking information,
-if it exists, for the current branch.
+Running `git checkout` without specifying a branch will output the
+tracking information, if it exists, for the current branch, but it's
+slower than getting the same information from `git status`.
 
 `git checkout (-b|-B) <new-branch> [<start-point>]`::