<branch> can be omitted in this syntax, and it's actually documented a
few paragraphs down:

  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 exists, for the current branch.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
---
 Documentation/git-checkout.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Documentation/git-checkout.txt b/Documentation/git-checkout.txt
index 99c8c0dc0f..28817cfa41 100644
--- a/Documentation/git-checkout.txt
+++ b/Documentation/git-checkout.txt

@@ -23,7 +23,7 @@ or the specified tree.  If no paths are given, 'git checkout' will
 also update `HEAD` to set the specified branch as the current
 branch.
 
-'git checkout' <branch>::
+'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

-- 
2.21.0.479.g47ac719cd3