[css-sizing-5][editorial] Add/tweak notes in interpolation-size.

Author: tabatkins

css-values-5/Overview.bs

@@ -3983,15 +3983,33 @@ Automatically interpolating sizing keywords: the 'interpolate-size' property</h3
 			as though it is ''calc-size(<var>keyword</var>, size)''
 			and applying the rules in [[#interpolate-calc-size]].
 			In other cases,
-			an <<size-keyword>> still cannot be interpolated.
+			a <<size-keyword>> still cannot be interpolated.
 		</dd>
 	</dl>
 
-	Note: If we had a time machine, this property wouldn't exist.
+	Note: Setting ''allow-keywords'' allows <em>most</em> size values to be interpolated,
+	but not all;
+	different keywords are still not interpolable with each other,
+	as layout algorithms can change their behavior
+	based on precisely which sizing keyword a descendant is using,
+	in a way that can't be represented during interpolation.
+
+	Note: Setting ''allow-keywords'' <em>mostly</em> removes the need to use ''calc-size()'' manually,
+	but there are still some cases that can benefit from using it.
+	For example, if the [=containing block=] size is [=indefinite=],
+	naively interpolating ''width: 100%'' to ''width: 0px'' won't work as expected
+	(since the intermediate values might [=behave as auto=],
+	making them the same size as ''100%''),
+	but manually writing ''width: calc-size(100%, size);'' and/or ''width: calc-size(any, 0px);'' will,
+	causing the element to smoothly interpolate between the two values.
+
+	Note: If we had a time machine, this property wouldn't exist;
+	the ''allow-keywords'' behavior would just be how things worked by default.
 	It exists because many existing style sheets assume that
 	the sizing keywords
 	(''width/auto'', ''width/min-content'', etc.)
-	cannot interpolate at all.
+	cannot interpolate at all,
+	and would produce broken behavior if they started interpolating.
 	For compatibility with this assumption,
 	allowing this interpolation requires an opt-in.
 	Because the property inherits,