[css-env-1] Actually dfn the term 'custom environment variable'.

Author: tabatkins

css-env-1/Overview.bs

@@ -78,9 +78,13 @@ A CSS <dfn export for=CSS>environment variable</dfn> is a name associated with a
 (a sequence of zero more CSS tokens, with almost no restrictions on what tokens can exist),
 similar to a [=custom property=].
 [=Environment variables=] can be defined by the user agent,
-or by the user.
-(In the latter case, the names are <<custom-property-name>>s,
-and start with `--` per standard for custom identifiers.)
+or by the author.
+
+If author-defined,
+they are a <dfn export>custom environment variable</dfn>.
+The name of a [=custom environment variable=]
+must conform to the <<custom-property-name>> grammar.
+This specification does not yet define how authors can define [=custom environment variables=].
 
 Issue: Is the set of UA-defined [=environment variables=] visible to script?
 If so, define an API on {{Document}} to expose them.