[css-display] Expand out informal definitions of BFC. Also fixes #1471.

Author: fantasai

css-display/Overview.bs

@@ -989,10 +989,18 @@ Appendix A: Glossary</h2>
 
 		<dt><dfn>BFC</dfn>
 		<dd>
-			Informal abbreviation for <a>block formatting context</a>.
-			Often used to refer to a <a>block formatting context</a> <em>root</em>,
-			that is, a <a>block box</a> that establishes a new <a>block formatting context</a>
-			for its contents.
+			Abbreviation for <a>block formatting context</a>.
+			Has various informal definitions referring to boxes which
+			contain internal floats, exclude external floats, and suppress margin collapsing,
+			and may refer specifically to:
+
+				* a <a>block container</a>
+					that establishes a new <a>block formatting context</a> for its contents
+				* a <a>block box</a> (i.e. a <a>block-level</a> block container)
+					that establishes a block formatting context for its contents
+					(as distinguished from a block box which does not)
+				* any block-level box that establishes a new formatting context
+					(other than an inline formatting context)
 
 		<dt><dfn>out-of-flow</dfn>
 		<dt><dfn>in-flow</dfn>