add comment based on comment on PR #10304 by Jason Malinowski

src/Compilers/Core/Portable/Compilation/ParseOptions.cs

@@ -38,6 +38,9 @@ public ParseOptions WithKind(SourceCodeKind kind)
             return CommonWithKind(kind);
         }
 
+        // It was supposed to be a protected implementation detail. 
+        // The "pattern" we have for these is the public With* method is the only public callable one, 
+        // and that forwards to the protected Common* like all the other methods in the class (see PR #10304 comments by Jason Malinowski) 
         [EditorBrowsable(EditorBrowsableState.Never)]
         public abstract ParseOptions CommonWithKind(SourceCodeKind kind);