Review Issue for Multi-column layout

[css-multicol-1] Add Accessibility Section (A11y Review)

Per [request for a11y-review](https://github.com/w3c/a11y-request/issues/2) of the [Multi-column Layout Module Level 1](https://www.w3.org/TR/css-multicol-1/#the-multi-column-model):

APA agrees with the concerns Rachel brought forth and has nothing more to add those concerns.

However, APA does request an Accessibility Considerations section be added, which acknowledges potential issues and references WCAG. Statement could read:

"Setting container height and line length can pose challenges for people with visual or cognitive disabilities. See [WCAG Success Criterion 1.4.10 Reflow](https://www.w3.org/WAI/WCAG21/Understanding/reflow.html) and [WCAG 1.4.8 Visual Presentation]( https://www.w3.org/WAI/WCAG21/quickref/#visual-presentation) to understand user needs."

At the end of Section 2 of the spec, a note is already present about accessibility issues occurring if the column's height exceeds the viewport height. Adding a page anchor at this point to lead down to the new Accessibility Concerns section would likely be helpful to folks exploring the spec. It could read:

"See Accessibility Concerns for additional details."

Also, please add the label `a11y-needs-resolution` to this issue.