Documentation/git-status: remove impossible porcelain status DR and DC

Commit 176ea74793 ("wt-status.c: handle worktree renames", 2017-12-27) made a porcelain status like .R or .C possible. They occur only when the source file is added to the index and the destination file is added with --intent-to-add. They also documented DR, but that status is impossible. The index change D means that the source file does not exist in the index. The worktree change R/C states that the file has been renamed/copied since the index, but that's impossible if it did not exist there. Reported-by: Elijah Newren <newren@gmail.com> Signed-off-by: Johannes Altmanninger <aclopte@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>