[css-images-3] Clarify default sizing algorithm not used for replaced elements

[fantasai]

https://www.w3.org/TR/css-images-3/#concrete-size-resolution

The Default Sizing Algorithm isn't used for replaced elements (they have a more complex sizing algorithm that takes into account the min/max size constraints etc.), but this is not obvious from the spec so should be clarified.

[tabatkins]

Tho note that the DSA does give the expected behavior for various combinations of natural and specified sizes that the HTML spec uses for its elements, so we should verify if the current spec (which defers to 2.1) does the same.