Update natspec comments on state variables.

- changing some warnings to errors

test/libsolidity/syntaxTests/natspec/docstring_author_title_state_variable.sol

@@ -4,4 +4,5 @@ contract C {
     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.
+// DocstringParsingError: (17-56): Documentation tag @author not valid for non-public state variables.
+// DocstringParsingError: (17-56): Documentation tag @title not valid for non-public state variables.

test/libsolidity/syntaxTests/natspec/docstring_non_public_state_variable_with_return.sol

@@ -3,4 +3,4 @@ contract test {
   uint private state;
 }
 // ----
-// DocstringParsingError: (18-47): Documentation tag "@return" is only allowed on public state-variables.
+// DocstringParsingError: (18-47): Documentation tag @return not valid for non-public state variables.

test/libsolidity/syntaxTests/natspec/docstring_private_state_variable.sol

@@ -4,4 +4,4 @@ contract C {
     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.
+// DocstringParsingError: (17-74): Documentation tag @notice not valid for non-public state variables.

test/libsolidity/syntaxTests/natspec/docstring_variable.sol

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