[css-multicol] Introduction refers to tables for layout

[rachelandrew]

https://drafts.csswg.org/css-multicol-1/#introduction

In a workshop with authors, the current introduction to the specification was noted as seeming somewhat old-fashioned referring as it does to multicol being an alternative to using tables for layout. The group thought it would be more appropriate to refer to the unique behaviour of multicol rather than describing it as a tables alternative.

Current intro:

This module describes multi-column layout
in CSS. By using functionality described in this document, style sheets can declare that the content of an element is to be laid out in multiple columns.

On the Web, tables have also been used to describe multi-column layouts. The main benefit of using CSS-based columns is flexibility; content can flow from one column to another, and the number of columns can vary depending on the size of the viewport. Removing presentation table markup from documents allows them to more easily be presented on various output devices including speech synthesizers and small mobile devices.

[rachelandrew]

@frivoal @fantasai I would appreciate a review or any suggestions for improvement.