[clang][NFC] Update isAuxBuiltinID comment (#128038)

Clarify behavior of the function when the builtin is also supported on
the main target.
Based on feedback from llvm/llvm-project#126324

---------

Signed-off-by: Sarnie, Nick <[email protected]>

Author: sarnex

clang/include/clang/Basic/Builtins.h

@@ -408,7 +408,8 @@ class Context {
 
   unsigned getRequiredVectorWidth(unsigned ID) const;
 
-  /// Return true if builtin ID belongs to AuxTarget.
+  /// Return true if the builtin ID belongs exclusively to the AuxTarget,
+  /// and false if it belongs to both primary and aux target, or neither.
   bool isAuxBuiltinID(unsigned ID) const {
     return ID >= (Builtin::FirstTSBuiltin + NumTargetBuiltins);
   }