[css-syntax-3] Slightly rewrite the definition of 'declaration' to encompass its conceptual definition as well as its syntax dfn. Probably needs more work.

Author: tabatkins

css-syntax-3/Overview.bs

@@ -1671,13 +1671,18 @@ Parsing
 
 		declaration
 		
-			A declaration has a name,
+			Conceptually, declarations are a particular instance of
+			associating a property or descriptor name with a value.
+			Syntactically, a declaration has a name,
 			a value consisting of a list of component values,
 			and an important flag which is initially unset.
 
-			Declarations are further categorized as "properties" or "descriptors",
-			with the former typically appearing in qualified rules
-			and the latter appearing in at-rules.
+			Declarations are further categorized
+			as property declarations or descriptor declarations,
+			with the former setting CSS [=properties=]
+			and appearing most often in qualified rules
+			and the latter setting CSS [=descriptors=],
+			which appear only in at-rules.
 			(This categorization does not occur at the Syntax level;
 			instead, it is a product of where the declaration appears,
 			and is defined by the respective specifications defining the given rule.)