Documentation/checkout: clarify description git checkout can be used to switch branches and to retrieve files from the index or an arbitrary tree. Split the description into subsections corresponding to each mode to make each use easier to understand. Helped-by: Jeff King <peff@peff.net> Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>