builtin-rm.c: explain and clarify the "local change" logic

Explain the logic to check local modification a bit more in the comment,
especially because the existing comment that talks about "git rm --cached"
was placed in a part that was not about "--cached" at all.

Also clarify "if .. else if .." structure.

Signed-off-by: Junio C Hamano <gitster@pobox.com>