[css-display] Rewrite formatting context definition, correct some errors. ('flow' can sometimes establish a new formatting context, <display-internal> never establishes a new formatting context, exclusions can affect content across FC boundaries)

Author: fantasai

css-display/Overview.bs

@@ -741,23 +741,39 @@ Appendix A: Glossary</h2>
 
 		<dt><dfn>formatting context</dfn>
 		<dd>
-			A <a>formatting context</a> is an independent "unit" of layout.
-			When a box establishes a <a>formatting context</a>,
-			the layout of its descendants are not affected by any outside boxes,
-			and vice versa.
+			A <a>formatting context</a> is the environment
+			into which a set of related boxes are laid out.
+			Different formatting contexts lay out their boxes
+			according to different rules.
+			For example, a <a>flex formatting context</a>
+			lays out boxes according to the <a>flex layout</a> rules [[CSS3-FLEXBOX]],
+			whereas a <a>block formatting context</a>
+			lays out boxes according to the block-and-inline layout rules [[CSS21]].
+
+			When a box establishes a <em>new</em> formatting context
+			(whether that formatting context is of the same type as its parent or not),
+			it essentially creates a new, independent layout environment:
+			except through the sizing of the box itself,
+			the layout of its descendants is (generally)
+			not affected by the the rules and contents of
+			the formatting context outside the box, and vice versa.
 
 			<p class="example">
-				For example, when used among other ''flow'' boxes,
+				For example, in a <a>block formatting context</a>,
 				floated boxes affect the layout of surrounding boxes.
 				But their effects do not escape their <a>formatting context</a>:
 				the box establishing their <a>formatting context</a> grows to fully contain them,
-				and any other <a>formatting contexts</a> near them
-				shrink or move to avoid having the float overlap them.
+				and floats from outside that box
+				are not allowed to protrude into and affect the contents
+				inside the box.
 
-			Most <a>inner display types</a> establish formatting contexts automatically.
-			The ''flow'' layout mode is a notable exception,
-			with a choice between establishing a formatting context or not,
-			via the ''flow-root'' and ''flow'' 'display' values.
+			<p class="example">
+				As another example, margins do not collapse across formatting context boundaries.
+
+			<p class="note">
+				Exclusions are able to affect content across formatting context boundaries.
+				(At time of writing, they are the layout feature that can.)
+				[[CSS3-EXCLUSIONS]]
 
 		<dt><dfn>block formatting context</dfn>
 		<dt><dfn>inline formatting context</dfn>