[css-values-4] Re-add text defining the clamping of interpolation results. Fixes w3c#6097

Author: tabatkins

css-values-4/Overview.bs

@@ -448,6 +448,26 @@ Combining Values: Interpolation, Addition, and Accumulation</h2>
 	If a value type does not define a specific procedure for <a>accumulation</a>,
 	its <a>accumulation</a> operation is identical to <a>addition</a>.
 
+<h3 id="combining-range">
+Range Checking</h3>
+
+	Interpolation can result in a value outside the valid range for a property,
+	even if all of the inputs to interpolation are valid;
+	this especially happens when |p| is outside the [0, 1] range,
+	but some [=easing functions=] can cause this to occur even within that range.
+	If the final result
+	<em>after</em> interpolation, addition, and accumulation
+	is out-of-range for the target context the value is being used in,
+	it does not cause the declaration to be invalid.
+	Instead, the value must be clamped to the range allowed in the target context,
+	exactly the same as [=math functions=]
+	(see [[#calc-range]]).
+
+	Note: Even if interpolation results in an out-of-range value,
+	addition/accumulation might "correct" the result and bring it back into range.
+	Thus, clamping is only applied to the <em>final</em> result
+	of applying all interpolation-related operations.
+
 <h2 id="textual-values">
 Textual Data Types</h2>