[css-fonts-5] De-dup section headings.

Author: fantasai

css-fonts-5/Overview.bs

@@ -177,7 +177,7 @@ the 'size-adjust' descriptor</h3>
 <!-- should this stay in Fonts 4 or be here in Fonts 5? -->
 
 <h3 id="font-metrics-override-desc">
-Default font metrics overriding:
+Line Height Font Metrics Overrides:
 the 'ascent-override', 'descent-override', and 'line-gap-override' descriptors</h3>
 
 <pre class='descdef'>
@@ -281,8 +281,8 @@ within an individual element e.g. for characters not supported by the primary fo
 </div>
 
 <h3 id="font-sup-sub-override-desc">
-    Default font metrics overriding:
-    the 'superscript-position-override', 'subscript-position-override','superscript-size-override' and 'subscript-size-override' descriptors</h3>
+Superscript and subscript metrics overrides:
+the 'superscript-position-override', 'subscript-position-override','superscript-size-override' and 'subscript-size-override' descriptors</h3>
 
 Issue(5518):