directory-rename-detection.txt: small updates due to merge-ort optimizations

directory-rename-detection.txt: small updates due to merge-ort optimizations

In commit 0c4fd732f0 ("Move computation of dir_rename_count from
merge-ort to diffcore-rename", 2021-02-27), much of the logic for
computing directory renames moved into diffcore-rename.
directory-rename-detection.txt had claims that all of that logic was
found in merge-recursive.  Update the documentation.

Acked-by: Derrick Stolee <redacted>
Acked-by: Johannes Schindelin <redacted>
Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>