Document maybe_unused query improvement

Note that the unused-local and unused-static-variable queries no longer report variables marked with the C++17 maybe_unused attribute.

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

Author: jketema

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

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+The `cpp/unused-local-variable` and `cpp/unused-static-variable` queries no longer report variables marked with the C++17 `[[maybe_unused]]` attribute.