Fix documentation for is_valid_string_constraint

Make doxygen stop complaining about missing fields.

Author: romainbrenguier

src/solvers/refinement/string_constraint.h

@@ -89,6 +89,8 @@ std::string from_expr(
   const string_constraintt &expr);
 
 /// Checks the data invariant for string_constraintt
+/// \param stream : output stream
+/// \param [in] ns: namespace for `from_expr`
 /// \param [in] expr: the string constraint to check
 /// \return whether the constraint satisfies the invariant
 bool is_valid_string_constraint(