doc: git-checkout: clarify `git checkout <branch>`
From user feedback: several users commented that "Local modifications to the files in the working tree are kept, so that they can be committed to the <branch>." didn't seem accurate to them, since `git checkout <branch>` will often fail. One user also thought that "... and by pointing HEAD at the branch" was something that _they_ had to do somehow ("How do I point HEAD at a branch?") rather than a description of what the `git checkout` operation is doing for them. Explain when `git checkout <branch>` will fail and clarify that "pointing HEAD at the branch" is part of what the command does. 6 users commented that the "You could omit <branch>..." section is extremely confusing. Explain this in a much more direct way. Signed-off-by: Julia Evans <julia@jvns.ca> Signed-off-by: Junio C Hamano <gitster@pobox.com>