CPP: Add change notes for the non-returning function improvements.

Author: alexet

cpp/ql/lib/change-notes/2023-09-08-more-unreachble.md

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+---
+category: minorAnalysis
+---
+* Functions that do not return due to calling functions that don't return (e.g. `exit`) are now detected as
+ non returning in the IR and dataflow.

cpp/ql/src/change-notes/2023-09-08-unreachble-edges.md

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+---
+category: minorAnalysis
+---
+* The queries `cpp/double-free` and `cpp/use-after-free` find fewer false positives
+  in cases where a non-returning function is called.