[css-env-1] Specify argument grammar for env() function (w3c#13642)

* Add argument grammar for env() function

* Add missing explanation

Author: tursunova

css-env-1/Overview.bs

@@ -291,6 +291,16 @@ use the ''env()'' function:
 	<dfn function>env()</dfn> = env( <<custom-ident>> <<integer [0,∞]>>*, <<declaration-value>>? )
 </pre>
 
+The ''env()'' function's [=argument grammar=] is:
+
+<pre class=prod>
+	<dfn><<env-args>></dfn> = env( <<declaration-value>>, <<declaration-value>>? )
+</pre>
+
+Like ''var()'', a bare comma can be used with nothing following it,
+indicating that the second <<declaration-value>> was passed,
+just as an empty sequence.
+
 The ''env()'' function can be used in place of any part of a value in any property on any element,
 or any part of a value in any descriptor on any [=at-rule=],
 and in several other places where CSS values are allowed.