[css-font-loading] Clarify some language. Not super-happy, but it's better.

Author: tabatkins

css-font-loading/Overview.bs

@@ -175,7 +175,9 @@ The <code>FontFace</code> Interface</h2>
 
 	All <a interface>FontFace</a> objects also contain
 	internal <dfn attribute for=FontFace>[<!---->[Urls]]</dfn> and <dfn attribute for=FontFace>[<!---->[Data]]</dfn> slots,
-	of which one is not <code>null</code> and the rest are <code>null</code>.
+	of which one is <code>null</code> and the other is not <code>null</code>
+	(the non-null one is set by the constructor,
+		based on which data is passed in).
 
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css-font-loading/Overview.html

@@ -305,7 +305,9 @@ <h2 class="heading settled heading" data-level=2 id=fontface-interface><span cla
 
 <p>All <a class=idl-code data-link-type=interface href=#dom-fontface title=FontFace>FontFace</a> objects also contain
 	internal <dfn class=idl-code data-dfn-for=FontFace data-dfn-type=attribute data-export="" id=dom-fontface-urls>[[Urls]]<a class=self-link href=#dom-fontface-urls></a></dfn> and <dfn class=idl-code data-dfn-for=FontFace data-dfn-type=attribute data-export="" id=dom-fontface-data>[[Data]]<a class=self-link href=#dom-fontface-data></a></dfn> slots,
-	of which one is not <code>null</code> and the rest are <code>null</code>.</p>
+	of which one is <code>null</code> and the other is not <code>null</code>
+	(the non-null one is set by the constructor,
+		based on which data is passed in).</p>