[css-containment] Clarify "must be" statements

[frivoal]

In a few of places, css-containment uses the phrasing "the element must be a <foo>",
where <foo> ∈ { formatting context, stacking context}.

First, this is ambiguous. While it is meant as a requirement that the UA changes the element into being a <foo> if it isn't one already, it could be read as implying the the effect under discussion does not apply if the element isn't already a <foo>.

Moreover, when the element isn't a &ls;foo>, it doesn't say how to make it into one. When <foo> is a stacking context, this is fine, as there no ambiguity how turn into one of these, but for formatting context it is a problem, as there's multiple ways an element could be become one, and the spec doesn't say so.

[frivoal]

I've created a pull request (#831) to address this. Feedback welcome.