Small clarity improvements to canvas transform methods

While in the area for #2845, I noticed the following small tweaks that we could make to improve that section of the spec:

• Replace "abort these steps" with "return" everywhere. (The latter is a more modern convention, per Infra.)
• Link "multiplying" in the definition of transform(a, b, c, d, e, f) to https://drafts.fxtf.org/geometry/#matrix-multiply
• Re-do setTransform(a, b, c, d, e, f) steps 2-3 to reset the current transformation matrix to the appropriate matrix (a c e / b d f / 0 0 1), instead of resetting the matrix to the identity matrix then invoking the transform() method. It's bad practice to invoke methods from other methods.

This is a good first bug :)