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

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>