docs(changelog): record GH #5352 Mutex atomic_flag::test fix

PR #5353 (cherry-picked as 7a14d52) closed issue #5352. Both were
added to the 1.15.3 milestone after the release commit landed, so the
CHANGELOG and doc/99100-ReleaseNotes.page entries were missing.

Author: matejk

CHANGELOG

@@ -62,6 +62,7 @@ Features, Enhancements and Third Party Updates:
 
 Bug Fixes and Improvements:
 
+- GH #5352 C++20 does not guarantee atomic_flag::test (PR #5353)
 - GH #5347 PDF does not build with make
 - GH #5342 Oracle ODBC CI fail
 - GH #5340 PropertyFileConfiguration does not persist deletion of last key from included property files

doc/99100-ReleaseNotes.page

@@ -42,6 +42,7 @@ Bundled Third-Party Library Upgrades:
 
 !!Bug Fixes and Improvements
 
+  - GH #5352 C++20 does not guarantee atomic_flag::test (PR #5353)
   - GH #5347 PDF does not build with make
   - GH #5342 Oracle ODBC CI fail
   - GH #5340 PropertyFileConfiguration does not persist deletion of last key from included property files