Refine the grammar of `field-name` in `defstruct`.

[shhyou]

The defstruct[*] form does support field options. Therefore use the same grammar as [define-]struct in the documentation.

[racket-discourse-github-bot]

This pull request has been mentioned on Racket Discourse. There might be relevant details there:

https://racket.discourse.group/t/documenting-struct-in-scribble/2618/3

[sorawee]

This looks good to me!

[shhyou]

Thanks a lot for reviewing!

I don't have push access. @samth could you help merging?

[sorawee]

Merged. Thanks!