Documentation/RelNotes: improve release note for rename detection work

Documentation/RelNotes: improve release note for rename detection work

There were some early changes in the 2.31 cycle to optimize some setup
in diffcore-rename.c[1], some later changes to measure performance[2],
and finally some significant changes to improve rename detection
performance.  The final one was merged with the note

   Performance optimization work on the rename detection continues.

That works for the commit log, but feels misleading as a release note
since all the changes were within one cycle.  Simplify this to just

   Performance improvements for rename detection.

The former wording could be seen as hinting that more performance
improvements will come in 2.32, which is true, but we can just cover
those in the 2.32 release notes when the time comes.

[1] a5ac31b5b1 (Merge branch 'en/diffcore-rename', 2021-01-25)
[2] d3a035b055 (Merge branch 'en/merge-ort-perf', 2021-02-11)
[3] 12bd17521c (Merge branch 'en/diffcore-rename', 2021-03-01)

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