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

Author: Pierre-Sassoulas

changelog/14523.improvement.rst

@@ -0,0 +1,7 @@
+The assertion comparison helpers and the truncator now hand the
+explanation between them as an iterator, so the dispatcher only has to
+materialise lines the truncator actually keeps. For the typical small
+diff this is a no-op; for the rare case where two large collections
+fail to compare, peak memory drops by a few percent because the
+millions of lines that would have been built and immediately discarded
+are no longer built.