[css-transforms] Clarify definition of <transform-function>

The <transform-function> value is described as list of transform
functions.[1] To be more precise it should be mentioned that it is a
*space-separated* list.
Also, it is not explicitly mentioned whether functions can appear
multiple times within the list.

Instead of defining this as text, it may be better to define the
syntax using the value definition syntax[2].

Sebastian

[1] https://drafts.csswg.org/css-transforms/#typedef-transform-function
[2] https://drafts.csswg.org/css-values-3/#value-defs