[css-sizing-3] max-content sizes of replaced elements errors

[fantasai]

https://drafts.csswg.org/css-sizing-3/#intrinsic-sizes which discusses the max-content sizes of replaced elements is a bit off in a few ways:

• The conditional for when this text triggers hooks the text into the undefined cases in CSS2.1, but the text defines a bunch of cases to which the conditional doesn't apply.
• The conditional is above bolded text, so it's not obvious that it limits the text below it.
• The text claims to apply to boxes with / without preferred aspect ratio, but it really should apply specifically to replaced elements.
• The first bullet point of the last case can't apply under the conditional, and is non-Web-compatible otherwise, so should be deleted.

Original commit of this section: c42c80f

[fantasai]

@cbiesinger @dholbert Mind reviewing the updated section? :)

[cbiesinger]

In this line:

Note: Note that when the box has a [=preferred aspect ratio=],

I would remove the "Note that" as it's repetitive.

Otherwise, looks good, except I have no idea if removing 300x150 from boxes with an aspect ratio is web compatible.

[dholbert]

Looks good to me.