[css-transforms-2] Explicitly specify that a transform is a containing block for all descendants

[chrishtr]

Currently, it's either unspecified or unclear whether a transform induces a containing block
for all children, including fixed-position. css-transforms-1 has this text, and it needs to be
added to css-transforms-2. Discussed with @smfr and there is agreement on this point.

https://www.w3.org/TR/css-transforms-1/#transform-property

[RByers]

Dupe of #913