[css-transforms-2] Limit effects of preserve-3d to transformable elements.

This clarifies an earlier resolution by limiting the effect of
transform-style: preserve-3d to transformable elements, matching the way
that resolution has been implemented in both Gecko and Chromium.

Fixes #6430.

Author: dbaron

css-transforms-2/Overview.bs

@@ -794,7 +794,12 @@ Used value: flat if a <a href="#grouping-property-values">grouping property</a>
 Animation type: discrete
 </pre>
 
-A computed value of ''transform-style/preserve-3d'' for 'transform-style' establishes both a stacking context and a [=containing block for all descendants=]. If the used value is ''transform-style/preserve-3d'' then it also establishes or extends a <a>3D rendering context</a>.
+A computed value of ''transform-style/preserve-3d'' for 'transform-style'
+on a <a>transformable element</a>
+establishes both a stacking context and
+a [=containing block for all descendants=].
+If the used value is ''transform-style/preserve-3d''
+then it also establishes or extends a <a>3D rendering context</a>.
 
 Grouping property values {#grouping-property-values}
 ------------------------