Clarify definition of the REC-track document

This makes it clear that a document that uses the various publication
types and transitions of the REC-track is indeed a REC-track document, even
if it isn't planning to go all the way to the end.

Some groups for instance have a declared intention of only going as far
as CR. That doesn't make the documents on the "CR track", as there's no
such thing.

See w3c/strategy#438 (comment)

index.bs

@@ -3196,6 +3196,9 @@ The W3C Recommendation Track</h3>
 		<li>Publication as a [=W3C Recommendation=].
 	</ol>
 
+	A <dfn lt="W3C Recommendation Track document | REC Track document | Recommendation Track document | Recommendation-track document">W3C Recommendation Track document</dfn>
+	is any document whose current status is one of the five in the numbered list above.
+
 	<div role="region" aria-label="Basic W3C Recommendation Track">
 		<pre class=include-raw>
 			path: basic-rec-track.svg