Update syntax guidelines to mention N parameter case and callback functions

[Elchi3]

As worked out in #4136, this PR updates the Syntax section guideline to talk about the cases for N parameters and callback functions. Also removes types from the slice example given we don't want to have types (yet).

[github-actions]

Preview URLs

• https://pr4249.content.dev.mdn.mozit.cloud/en-US/docs/MDN/Structures/Syntax_sections

Flaws

URL: /en-US/docs/MDN/Structures/Syntax_sections
Title: Syntax sections
on GitHub

No flaws! 🎉

External URLs

URL: /en-US/docs/MDN/Structures/Syntax_sections
Title: Syntax sections
on GitHub

• https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_form (1 time)

[hamishwillee]

Excellent. Thanks