[css-grid] Break line, clarify that unsupported overflow means visible.

Author: fantasai

css-grid/Overview.bs

@@ -27,6 +27,7 @@ Ignored Terms: containing block, <positive-integer>, <ident>, auto, grid-*-start
 Ignored Vars: A, B, C, size-contribution, track-sizes, extra-space
 Link Defaults: css21 (property) margin/min-height/max-height/min-width/max-width, css-align-3 (dfn) alignment container/alignment subject/box alignment properties, css-align-3 (value) stretch/baseline, css-position-3 (property) left, css-position-3 (property) position, css-writing-modes-3 (dfn) start/end
 At Risk: the ''display/subgrid'' value
+At Risk: 'overflow' applied to subgrids
 At Risk: the <<percentage>> value in 'grid-row-gap'/'grid-column-gap'
 At Risk: application of grid placement to absolutely-positioned boxes
 </pre>
@@ -2319,13 +2320,15 @@ Subgrids</h2>
 			as are any specified width/height constraints.
 
 		<li>
-			Layoutwise, the <a>subgrid</a>’s <a>explicit grid</a> is always aligned with the corresponding section of the parent <a>grid</a>;
+			Layoutwise, the <a>subgrid</a>’s <a>explicit grid</a>
+			is always aligned with the corresponding section of the parent <a>grid</a>;
 			the 'align-content'/'justify-content' properties on it are also ignored.
 
 		<li>
 			The UA <em>may</em> apply 'overflow' to <a>subgrids</a>
 			so the contents of the subgrid can be scrolled aside.
 			(Note: the act of scrolling does not affect layout.)
+			Overflow must otherwise be treated as ''visible''.
 
 		<li>
 			Since <a>subgrids</a> can be placed before their contents are placed,