lgc: document a potentially surprising value overload

(cherry picked from commit 521f6d688e02d6766155a3f7dc5ae737efcf6eed)

Author: nhaehnle

lgc/interface/lgc/Pipeline.h

@@ -185,7 +185,7 @@ enum InvariantLoadsOption : unsigned { Auto = 0, EnableOptimization = 1, Disable
 // Struct with the information for one color export
 struct ColorExportInfo {
   unsigned hwColorTarget;
-  unsigned location;
+  unsigned location; // Overloaded to a bitmask for mrtz exports
   bool isSigned;
   llvm::Type *ty;
 };