[css-transforms] properly define syntax of transform-origin, perspective, perspective-origin SVG presentation attributes

[dbaron]

The edits for #2623 removed a section on SVG Data Types that was previously referenced when defining the syntax of all of the newly-introduced SVG presentation attributes, not just transform. I think something like this section is probably still needed for defining transform-origin (in both levels 1 and 2), perspective (in level 2), and perspective-origin, unless there's general wording that covers this.

There's also still a broken link to "SVG Data Types" in level 2 of the spec that need to be fixed.