RelNotes 2.18: clarify where directory rename detection applies

Mention that this feature works with some commands (merge and cherry-pick,
implying that it also works with commands that build on these like rebase
-m and rebase -i).  Explicitly mentioning two commands hopefully implies
that it may not always work with other commands (am, and rebase without
flags that imply either -m or -i).

Also, since the directory rename detection from this cycle was
specifically added in merge-recursive and not diffcore-rename, remove the
'in "diff" family" phrase from the note.  (Folks have requested in the
past that `git diff` detect directory renames and somehow simplify its
output, so it may be helpful to avoid implying that diff has any new
capability here.)

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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