[css-position-3][editorial] Add more subheadings.

Author: fantasai

css-position-3/Overview.bs

@@ -694,6 +694,11 @@ Absolute (and Fixed) Positioning</h3>
 	into which it is sized and positioned
 	by the specified amounts.
 	The resulting rectangle is called the <dfn export>inset-modified containing block</dfn>.
+	(For disambiguation, the actual [=containing block=] of an [=absolutely positioned box=]
+	can also be called the <dfn export>absolute-position containing block</dfn>.)
+
+<h4 id="resolving-insets">
+Resolvings Insets: the “Inset-Modified Containing Block”</h4>
 
 	If only one [=inset property=] in a given axis is ''top/auto'',
 	it is set to zero.
@@ -703,12 +708,12 @@ Absolute (and Fixed) Positioning</h3>
 	<dl class=switch>
 		<dt>for ''align-self/self-start'' alignment or its equivalent
 		<dd>
-			Set its start-edge [=inset property=] to the static position,
+			Set its start-edge [=inset property=] to the [=static position=],
 			and its end-edge [=inset property=] to zero.
 
 		<dt>for ''align-self/self-end'' alignment or its equivalent
 		<dd>
-			Set its end-edge [=inset property=] to the static position,
+			Set its end-edge [=inset property=] to the [=static position=],
 			and its start-edge [=inset property=]  to zero.
 
 		<dt>for ''align-self/center'' alignment
@@ -758,7 +763,7 @@ Absolute (and Fixed) Positioning</h3>
 
 
 <h4 id="staticpos-rect">
-Resolving Automatic Insets: the “Static-Position Rectangle”</h4>
+Calculating the Static Position and the “Static-Position Rectangle”</h4>
 
 	When both [=inset properties=] in a given axis are ''left/auto'',
 	they are resolved into a <dfn export>static position</dfn>