Add more explanations/links to Builder

[ehsanmok]

Hi

Following up on the builder tutorial, I found the transition to 06-optional-field to the end is tough. It'd help a lot to explain a little in more details what to do or link to similar code about how to check whether Option exists or not in 06.

[dtolnay]

Thanks! I have added some more detail to 06-optional-field. Please let me know if it is still unclear how I expect for that part of the macro to work.