Natspec: Don't copy from base function if return parameters differ

test/libsolidity/syntaxTests/natspec/invalid/return_param_amount_differs.sol

@@ -0,0 +1,14 @@
+interface IThing {
+    /// @return x a number
+    /// @return y another number
+    function value() external view returns (uint128 x, uint128 y);
+}
+
+contract Thing is IThing {
+    struct Value {
+        uint128 x;
+        uint128 y;
+    }
+
+    Value public override value;
+}

test/libsolidity/syntaxTests/natspec/invalid/return_param_amount_differs2.sol

@@ -0,0 +1,15 @@
+interface IThing {
+    /// @param v value to search for
+    /// @return x a number
+    /// @return y another number
+    function value(uint256 v) external view returns (uint128 x, uint128 y);
+}
+
+contract Thing is IThing {
+    struct Value {
+        uint128 x;
+        uint128 y;
+    }
+
+    mapping(uint256=>Value) public override value;
+}