Factor out "when it generates a box"

[zcorpan]

See #4143 (comment)

when it generates a box (i.e., is not display: none or display: contents)

This phrase is repeated a couple of times in the rendering section. It's necessary because we don't want to require things like "behave as display: inline-block" on elements that are display: none. But maybe it should be factored out, so it reads better and is defined once.

[annevk]

Ideally @tabatkins, @fantasai, et al. define this in the relevant CSS module.

[tabatkins]

You can just say generates a <a>box</a> (where that links to https://www.w3.org/TR/css-display-3/#css-box). Nothing more complex needs to be said.