Allow natspec comments on state variables.

test/libsolidity/syntaxTests/natspec/docstring_author_title_state_variable.sol

@@ -0,0 +1,7 @@
+contract C {
+    /// @title title
+    /// @author author
+    uint private state;
+}
+// ----
+// Warning: (17-56): Documentation tag @title and @author is only allowed on contract definitions. It will be disallowed in 0.7.0.

test/libsolidity/syntaxTests/natspec/docstring_non_public_state_variable_with_return.sol

@@ -0,0 +1,6 @@
+contract test {
+  /// @return returns something
+  uint private state;
+}
+// ----
+// DocstringParsingError: (18-47): Documentation tag "@return" is only allowed on public state-variables.

test/libsolidity/syntaxTests/natspec/docstring_private_state_variable.sol

@@ -0,0 +1,7 @@
+contract C {
+    /// @notice example of notice
+    /// @dev example of dev
+    uint private state;
+}
+// ----
+// Warning: (17-74): Documentation tag on non-public state variables will be disallowed in 0.7.0. You will need to use the @dev tag explicitly.

test/libsolidity/syntaxTests/natspec/docstring_state_variable.sol

@@ -1,9 +1,5 @@
 contract C {
-    /// @title example of title
-    /// @author example of author
     /// @notice example of notice
     /// @dev example of dev
-    /// @param example of param
-    /// @return example of return
     uint public state;
-}
+}

test/libsolidity/syntaxTests/natspec/docstring_state_variable_too_many_return_tags.sol

@@ -0,0 +1,9 @@
+contract test {
+  /// @notice example of notice
+  /// @dev example of dev
+  /// @return returns something
+  /// @return returns something
+  uint public state;
+}
+// ----
+// DocstringParsingError: (18-137): Documentation tag "@return" is only allowed once on state-variables.

test/libsolidity/syntaxTests/natspec/docstring_variable.sol

@@ -11,3 +11,4 @@ contract C {
     }
 }
 // ----
+// Warning: (290-295): Only state variables can have a docstring. This will be disallowed in 0.7.0.