Document unused-variable initializer side-effect fix

Add a minor analysis change note for suppressing alerts on variables whose initializers may have side effects.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jketema

cpp/ql/src/change-notes/2026-10-29-unused-variable-initializers.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+The `cpp/unused-local-variable` and `cpp/unused-static-variable` queries no longer report variables whose initializers may have side effects.