Documentation/git-status: clarify status table for porcelain mode

It is possible to have the output ' A' from 'git status --porcelain' by adding a file using the '--intend-to-add' flag. Make this clear by adding the pattern in the table of the documentation. However the mode 'DM' (deleted in the index, modified in the working tree) is not possible in the non-merge case in which the file only shows as 'D ' (and adding it back to the worktree would show an additional line of an '??' untracked file). It is also not possible in the merge case as then the mode involves a 'U' on one side of the merge. Remove that pattern. Reported-by: Ross Light <light@google.com> Signed-off-by: Stefan Beller <sbeller@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>