[css-syntax-3] Slightly rewrite the definition of 'declaration' to en…

…compass its conceptual definition as well as its syntax dfn. Probably needs more work.

css-syntax-3/Overview.bs

@@ -1671,13 +1671,18 @@ Parsing</h2>
 
 		<dt><dfn id="declaration">declaration</dfn>
 		<dd>
-			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 <var>important</var> flag which is initially unset.
 
-			Declarations are further categorized as "properties" or "descriptors",
-			with the former typically appearing in <a>qualified rules</a>
-			and the latter appearing in <a>at-rules</a>.
+			Declarations are further categorized
+			as <dfn>property declarations</dfn> or <dfn>descriptor declarations</dfn>,
+			with the former setting CSS [=properties=]
+			and appearing most often in <a>qualified rules</a>
+			and the latter setting CSS [=descriptors=],
+			which appear only in <a>at-rules</a>.
 			(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.)