[css-values] Add note callign out the fact that unitless-0 doesn't work in calc().

Author: tabatkins

css-values/Overview.bs

@@ -1669,6 +1669,13 @@ Type Checking</h4>
 	(''cm'' is <<length>>, ''deg'' is <<angle>>, etc.).
 	An ''attr()'' expression's type is given by its <<type-or-unit>> argument.
 
+	Note: Because <<number-token>>s are always interpreted as <<number>>s or <<integer>>s,
+	"unitless 0" <<length>>s or <<angle>>s aren't supported in ''calc()''.
+	That is, ''width: calc(0 + 5px);'' is invalid,
+	even though both ''width: 0;''
+	and ''width: 5px;''
+	are valid.
+
 	If percentages are accepted in the context in which the expression is placed,
 	a <<percentage-token>> has the type of the value that percentages are relative to.
 	For example, in the 'width' property, percentages have the <<length>> type,