reword to make less promises

unified diff is not "fast" just "less likely slow" or "slow in different circumstances". so this doesn't ensure the problem never happens, it just reduces its chance.
python · jdevries3133 · Dec 17, 2018 · Dec 18, 2018 · Dec 18, 2018 · Jul 22, 2021
commit adc7d6f92818b3e64a652ff290d900b1dfae0caf
diff --git a/Misc/NEWS.d/next/Library/2021-07-21-22-57-51.bpo-19217.vm-cr-.rst b/Misc/NEWS.d/next/Library/2021-07-21-22-57-51.bpo-19217.vm-cr-.rst
@@ -3,7 +3,7 @@ items. Based on an internal heuristic, the algorithm used to produce the diff
 method will switch based on the relevant magnitude of inputs. As a result, diff
 output after a failing assertion may appear differently for large inputs.
 Specifically, unittest will internally switch from using :func:`difflib.ndiff`
-(slow) to using :func:`difflib.unified_diff` (fast).
+(slow) to using :func:`difflib.unified_diff` (less likely to be slow).
 
-This optimization ensures that the non-linear time complexity of
-:func:`difflib.ndiff` does not cause a test suite to hang.
+This optimization reduces that chance that non-linear time complexity of
+diff algorithms do not cause a test suite's failing test to hang.