[css-inline-3] Add 1em distance to ideographic heuristics.

Author: fantasai

css-inline-3/Overview.bs

@@ -2672,13 +2672,20 @@ Appendix A: Synthesizing Alignment Metrics</h2>
 
 			<ol>
 				<li>
-					The [=central baseline=] is expected to be
+					The [=central baseline=] is defined to be
 					halfway between the [=ideographic-over=] and [=ideographic-under=] baselines,
 					so any two of these determines the third.
 				<li>
-					In CJK fonts the [=ascent=] and [=descent=] often match
+					The [=ideographic-over=] and [=ideographic-under=] baselines
+					are typically ''1em'' apart,
+					so if only one of the
+					[=ideographic-over=]/[=ideographic-under=]/[=ideograph-central=]
+					baselines are provided,
+					this relation can be used to calculate the other two.
+				<li>
+					In CJK fonts the [=ascent=] and [=descent=] typically match
 					the [=ideographic-over=] and [=ideographic-under=] baselines,
-					so can be used as a fallback when those values are missing.
+					so can be used as a fallback when both are missing.
 			</ol>
 
 		<dt>Measure the font