[css-easing-1] Add range annotation to cubic-bezier function syntax definition

Fixes w3c#4960.

Author: birtles

css-easing-1/Overview.bs

@@ -154,7 +154,7 @@ A <a>cubic B&eacute;zier easing function</a> has the following syntax
 <div class="prod"><dfn type>&lt;cubic-bezier-easing-function&gt;</dfn> =
   ''ease'' | ''ease-in'' | ''ease-out'' | ''ease-in-out'' |
   <span class="atom"><a lt="cubic-bezier()"
-  function>cubic-bezier</a>(<<number>>, <<number>>, <<number>>,
+  function>cubic-bezier</a>(<<number [0,1]>>, <<number>>, <<number [0,1]>>,
   <<number>>)</span></div>
 
 The meaning of each value is as follows:
@@ -169,7 +169,7 @@ The meaning of each value is as follows:
 ::  Equivalent to ''cubic-bezier(0, 0, 0.58, 1)''.
 :   <dfn>ease-in-out</dfn>
 ::  Equivalent to ''cubic-bezier(0.42, 0, 0.58, 1)''.
-:   <dt><dfn function lt="cubic-bezier()">cubic-bezier(<<number>>, <<number>>, <<number>>, <<number>>)</dfn></dt>
+:   <dt><dfn function lt="cubic-bezier()">cubic-bezier(<<number [0,1]>>, <<number>>, <<number [0,1]>>, <<number>>)</dfn></dt>
 ::  Specifies a <a>cubic B&eacute;zier easing function</a>.
     The four numbers specify points <var>P1</var> and <var>P2</var> of
     the curve as (<var ignore>x1</var>, <var ignore>y1</var>, <var