[css-display] Export terminology for establishing a FC without saying…

… which

(Follow up on 1457)

css-display-3/Overview.bs

@@ -1055,6 +1055,28 @@ Appendix A: Glossary</h2>
 			<p class="example">
 				As another example, margins do not collapse across [=formatting context=] boundaries.
 
+			Specifications may call for an element or box to
+			<dfn export lt="establishes a formatting context | establish a formatting context | establish a new formatting context | establishes a new formatting context">establish a formatting context</dfn>
+			without further precision as to the type of formatting context or how to do so.
+			This operation is not applicable to
+			non-atomic inline-level boxes
+			nor to boxes with a layout-internal display-type.
+			Specifications that invoke this
+			must either exclude such boxes,
+			or to blockify them first.
+			If the box is a block container that does not establish a BFC,
+			it is made to establish a BFC.
+			Otherwise, the box already establishes a formatting context, and this operation is a no-op.
+			This has no effect on the computed value of the display property.
+
+			Note: If a specification defines a new type of box which is neither
+			non atomic inline level,
+			layout-internal,
+			block containers,
+			or some other display type that establishes a FC,
+			then that specification must define the effect,
+			of invoking “<a>establishes a formatting context</a>” for that type of box.
+
 			<p class="note">
 				Exclusions are able to affect content across [=formatting context=] boundaries.
 				(At time of writing, they are the only layout feature that can.)