Add an entry to the Release Notes

Author: NagyDonat

clang/docs/ReleaseNotes.rst

@@ -464,6 +464,11 @@ Crash and bug fixes
 Improvements
 ^^^^^^^^^^^^
 
+- The checker option ``optin.cplusplus.VirtualCall:PureOnly`` was removed,
+  because it had been deprecated since 2019 and it has no valid usecase (it had
+  been kept for compatibility with the pre-2019 status, setting it to true is
+  equivalent to completely disabling the checker).
+
 Moved checkers
 ^^^^^^^^^^^^^^