[css-scoping] Update to talk about slots/etc instead of content/distributed. Update ::slotted() section accordingly, and add section defining how slots work. Ref DOM a lot, since it has nearly all the terms we need now.

Author: tabatkins

css-scoping/Overview.bs

@@ -351,7 +351,7 @@ Shadow Hosts in a Shadow Tree</h4>
 <h4 id='slots-in-shadow-tree'>
 Slots and Slotted Elements in a Shadow Tree</h4>
 
-	<a>Slots</a> must act as if they were assigned ''display: contents'' in the <a>UA style sheet</a>.
+	<a>Slots</a> must act as if they were assigned ''display: contents'' in the <a>UA origin</a>.
 	This must be possible to override via 'display',
 	so they <em>do</em> generate boxes if desired.
 
@@ -485,7 +485,7 @@ Selecting Into the Light: the '':host'', '':host()'', and '':host-context()'' ps
 -->
 
 <h4 id='slotted-pseudo'>
-Selecting Shadow-Projected Content: the ''::slotted()'' pseudo-element</h4>
+Selecting Slot-Assigned Content: the ''::slotted()'' pseudo-element</h4>
 
 	The <dfn selector>::slotted()</dfn> pseudo-element
 	represents the elements <a>assigned</a> to a <a>slot</a>.
@@ -504,7 +504,7 @@ Selecting Shadow-Projected Content: the ''::slotted()'' pseudo-element</h4>
 	The ''::slotted()'' pseudo-element represents the elements that are:
 
 	* <a>assigned</a> to its <a>originating element</a>
-	* <a lt="match a selector against an element">matched</a> by the <<compound-selector>> argument
+	* <a lt="match a selector against an element">matched</a> by its <<compound-selector>> argument
 
 	<div class="example">
 		For example, say you had a component with both children and a shadow tree,