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>

Documentation/RelNotes/2.18.0.txt

@@ -6,7 +6,7 @@ Updates since v2.17
 
 UI, Workflows & Features
 
- * Rename detection logic in "diff" family that is used in "merge" has
+ * Rename detection logic that is used in "merge" and "cherry-pick" has
    learned to guess when all of x/a, x/b and x/c have moved to z/a,
    z/b and z/c, it is likely that x/d added in the meantime would also
    want to move to z/d by taking the hint that the entire directory