C++: Add change note.

Author: jcreedcmu

change-notes/1.21/analysis-cpp.md

@@ -15,5 +15,6 @@
 | Memory may not be freed (`cpp/memory-may-not-be-freed`) | More correct results | Support added for more Microsoft-specific allocation functions, including `LocalAlloc`, `GlobalAlloc`, `HeapAlloc` and `CoTaskMemAlloc`. |
 | Memory is never freed (`cpp/memory-never-freed`) | More correct results | Support added for more Microsoft-specific allocation functions, including `LocalAlloc`, `GlobalAlloc`, `HeapAlloc` and `CoTaskMemAlloc`. |
 | Resource not released in destructor (`cpp/resource-not-released-in-destructor`) | Fewer false positive results | Resource allocation and deallocation functions are now determined more accurately. |
+| Comparison result is always the same | Fewer false positive results | The range analysis library is now more conservative about floating point values being possibly `NaN` |
 
 ## Changes to QL libraries