code: @Nonnull
      public BugInstance addVisitedField(PreorderVisitor visitor) {
          FieldAnnotation f = FieldAnnotation.fromVisitedField(visitor);
          addField(f);
          return this;
      }
docs: Add a field annotation for the field which is being visited by given
  visitor.
  
  @param visitor
  the visitor
  @return this object