[doc] Add terse changelog for streaming assertion comparisons

Author: Pierre-Sassoulas

changelog/14523.improvement.rst

@@ -1,8 +1,4 @@
-A dict comparison with many keys only on one side ("Left/Right contains
-N more items") no longer pretty-prints the whole extra subdict when the
-explanation is going to be truncated anyway: past the truncation budget
-it emits only the smallest few keys, one per line. The item count is
-still reported in the header and the truncation footer is unchanged, so
-no information the user would have seen is lost. A failing
-``assert {i: i for i in range(2000)} == {}`` no longer spends time
-formatting ~2000 lines only to discard them.
+Large assertion comparison diffs are now built lazily and capped to the
+truncation budget, so a huge ``==`` mismatch is no longer formatted in
+full just to be truncated. As a result the truncation footer no longer
+reports the exact number of hidden lines.