[css-color-hdr][editorial] Collect explanatory material together

Author: svgeesus

css-color-hdr-1/Overview.bs

@@ -134,7 +134,7 @@ Value Definitions {#values}
 	also accept the <a>CSS-wide keywords</a> as their property value.
 	For readability they have not been repeated explicitly.
 
-Controlling Dynamic Range {#controlling-dynamic-range}
+HDR Terminology {#terminology}
 ==========================
 
 Defining Dynamic Range {#defining-dynamic-range}
@@ -285,6 +285,9 @@ Introducing Headroom {#introducing-headroom}
 	to allow meaningful evaluation of the group of photos.
 </div>
 
+Controlling Dynamic Range {#controlling-dynamic-range}
+==========================
+
 The 'dynamic-range-limit' property {#the-dynamic-range-limit-property}
 -------------------------------