[css-values-4] Add some text clarifying how 'contexts' inherit into calculations.

Author: tabatkins

css-values-4/Overview.bs

@@ -4329,6 +4329,22 @@ Type Checking</h3>
 	and to make division-by-zero detectable at parse time.
 	This version now relaxes those restrictions.
 
+	[=Calculations=] are evaluated in terms of some "context",
+	depending on where they are placed,
+	which determines how units are resolved,
+	how percentages are treated,
+	etc.
+	By default, this context is defined by the property/descriptor/etc
+	that the calculation is being used in.
+	Functions can define a different context
+	for their contained values.
+	(For example, ''media-progress()'' evaluates its [=calculations=]
+	in the context of the [=media feature=] it mentions.)
+	Directly nested [=math functions=]
+	(forming a larger [=calculation tree=])
+	always use the context of their top-most [=math function=],
+	so the entire [=calculation tree=] is consistent.
+
 	<div algorithm>
 		To <dfn>determine the type of a [=calculation=]</dfn>: