[css-timing-1] Add syntax to section headings

[fantasai]

For sections that define a particular piece of syntax, Tab and I try to put the syntax in the heading so that they are easy to find in the Table of Contents, like this: https://drafts.csswg.org/css-values-3/#toc

I think the Timing Functions spec would benefit from this practice.

Also I would move the production definition from its own 2-line section at the end of the spec into section 2. Timing Functions. This way the syntax definition serves as an overview and ToC of the section.

[birtles]

Fixed by ee3b72b and 3da5bcf