[doc] Add changelog for streaming assertion comparisons (pytest-dev#14523)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Pierre-Sassoulas

changelog/14523.improvement.rst

@@ -0,0 +1,13 @@
+Assertion explanations are now built lazily and the truncator stops
+the comparison helpers as soon as it has enough output, so comparing
+two large collections no longer builds the full diff in order to
+discard it. A focused micro-benchmark the worst case scenario
+(``set(range(500_000)) == set(range(1, 500_001))``) drops from ~2,200 ms
+to ~43 ms; but realistic test suite with mostly small diffs should be
+unchanged.
+
+The truncation footer no longer reports the hidden-line count
+(``...Full output truncated (N lines hidden), ...`` becomes
+``...Full output truncated, ...``); diff lines now carry a redundant
+``\x1b[0m`` reset prefix (invisible to terminals) so we can handle
+line one by one.