[css-grid][css-flex] Terminology clean up

Grid and Flexbox used to need a sentence to say what kind of formatting
context grid/flex items would establish. Since that is now part of the
definition of "establishing a [new] formatting context", just link to
it.

This change is editorial.

css-flexbox-1/Overview.bs

@@ -638,8 +638,7 @@ Flex Items</h2>
 		Its contents will however inherit styles (such as font settings) from the flex container.
 	</div>
 
-	A <a>flex item</a> establishes a new formatting context for its contents.
-	The type of this formatting context is determined by its 'display' value, as usual.
+	A <a>flex item</a> <a>establishes a new formatting context</a> for its contents.
 	However, flex items themselves are <dfn>flex-level</dfn> boxes, not block-level boxes:
 	they participate in their container's flex formatting context,
 	not in a block formatting context.

css-grid-1/Overview.bs

@@ -1034,8 +1034,7 @@ Grid Items</h2>
 
 <h3 id="grid-item-display">
 	Grid Item Display</h3>
-	A <a>grid item</a> establishes a new formatting context for its contents.
-	The type of this formatting context is determined by its 'display' value, as usual.
+	A <a>grid item</a> <a>establishes a new formatting context</a> for its contents.
 	However, grid items are <dfn>grid-level</dfn> boxes, not block-level boxes:
 	they participate in their container's <a>grid formatting context</a>,
 	not in a block formatting context.