[css-typed-om] have a note saying when CSSTransformValue.toMatrix() doesn't work

[dbaron]

The definition of CSSTransformValue.toMatrix() has technical prose:

As the entries of such a matrix are defined relative to the px unit, if any <length>s in func involved in generating the matrix are not compatible units with px, throw a TypeError.

that says that the function will sometimes throw. But it's probably worth adding a note to make this clearer: it will happen when there is use of relative lengths or percentages in transform functions.

[tabatkins]

Done as part of 72d9e39