codegen: document _chopSignBits parameter of convertType function

libsolidity/codegen/CompilerUtils.h

@@ -130,6 +130,7 @@ public:
 	/// if a reference type is converted from calldata or storage to memory.
 	/// If @a _cleanupNeeded, high order bits cleanup is also done if no type conversion would be
 	/// necessary.
+	/// If @a _chopSignBits, the function resets the signed bits out of the width of the signed integer.
 	void convertType(Type const& _typeOnStack, Type const& _targetType, bool _cleanupNeeded = false, bool _chopSignBits = false);
 
 	/// Creates a zero-value for the given type and puts it onto the stack. This might allocate