[css-values] Clarify that syntactic range-checking is not performed on calc() values. Clarify that the used value is what's clamped.

Author: fantasai

css-values/Overview.bs

@@ -1521,8 +1521,10 @@ Computed Value</h4>
 <h4 id='calc-range'>
 Range Checking</h4>
 
-	The value resulting from an expression must be clamped to the
-	range allowed in the target context.
+	Parse-time range-checking of values is not performed within ''calc()'',
+	and therefore out-of-range values do not cause the declaration to become invalid.
+	However, the used value resulting from an expression
+	must be clamped to the range allowed in the target context.
 
 	Note: This requires all contexts accepting ''calc()''
 	to define their allowable values as a closed (not open) interval.