[css-display] Rephrase the definition of display:contents to refer to the Appendix, and be less awkward about pseudos.

Author: tabatkins

css-display/Overview.bs

@@ -478,26 +478,23 @@ Box Generation: the ''display/none'' and ''display/contents'' keywords</h3>
 			The element itself does not generate any boxes,
 			but its children and pseudo-elements still generate boxes as normal.
 			For the purposes of box generation and layout,
-			the element must be treated as if it had been replaced with its children and pseudo-elements in the document tree.
-			However, its ''::before'' and ''::after'' pseudo-elements still exist,
-			and are parented by the element's parent's principal box along with the rest of the element's box-generating contents.
+			the element must be treated as if it had been replaced in the [=element tree=]
+			by its contents
+			(including both its source-document children and its pseudo-elements,
+			such as ''::before'' and ''::after'' pseudo-elements,
+			which are generated before/after the element's children as normal).
 
 			Note: As only the box tree is affected,
 			any semantics based on the document tree,
 			such as selector-matching and event handling,
 			are not affected.
 
-			This value is treated as ''display: none'' on replaced elements,
-			and form controls
-			(such as <a href="https://html.spec.whatwg.org/multipage/forms.html#category-submit">submittable elements</a> in [[HTML]]).
-			The effect is exactly as if the UA style sheet had specified
-			''display: none !important'' on such elements,
-			except that the computed style is still ''contents''.
-
-			Note: Replaced elements and form controls are included in this special case
-			because their rendering requirements are often
-			outside the scope of the CSS formatting model,
-			and therefore removing only the element’s own generating box
+			This value behaves as ''display: none'' on replaced elements
+			and other elements whose rendering is not entirely controlled by CSS;
+			see [[#unbox]] for details.
+
+			Note: Replaced elements and form controls are treated specially
+			because removing only the element’s own generating box
 			is a more-or-less undefined operation.
 			As this behavior may be refined if use cases
 			(and more precise rendering models) develop,
@@ -506,7 +503,7 @@ Box Generation: the ''display/none'' and ''display/contents'' keywords</h3>
 
 		<dt><dfn>none</dfn>
 		<dd>
-			The element and its descendants generates no boxes.
+			The element and its descendants generate no boxes.
 	</dl>
 
 	Elements with either of these values do not have <a>inner</a> or <a>outer display types</a>,