Add release notes for pull request #150962,

Author: DeanSturtevant1

clang/docs/ReleaseNotes.rst

@@ -122,6 +122,17 @@ Improvements to Clang's diagnostics
   Added a new warning in this group for the case where the attribute is missing/implicit on
   an override of a virtual method.
 
+- Fixed an issue where emitted format-signedness diagnostics were not associated with an appropriate
+  diagnostic id. Besides being incorrect from an API standpoint, this was user visible, e.g.:
+  "format specifies type 'unsigned int' but the argument has type 'int' [-Wformat]"
+  "signedness of format specifier 'u' is incompatible with 'c' [-Wformat]"
+  This was misleading, because even though -Wformat is required in order to emit the diagnostics,
+  the warning flag the user needs to concerned with here is -Wformat-signedness, which is also
+  required and is not enabled by default. With the change you'll now see:
+  "format specifies type 'unsigned int' but the argument has type 'int', which differs in signedness [-Wformat-signedness]"
+  "signedness of format specifier 'u' is incompatible with 'c' [-Wformat-signedness]"
+  and the API-visible diagnostic id will be appropriate.
+  
 Improvements to Clang's time-trace
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