[css-shapes][editorial] Define production rule for <basic-shape> (#9727)

Author: cdoublev

css-shapes-1/Overview.bs

@@ -329,6 +329,11 @@ Basic Shapes</h2>
 
 	The <dfn>&lt;basic-shape&gt;</dfn> type
 	can be specified using basic shape functions.
+
+	<pre class=prod>
+		<<basic-shape>> = <<basic-shape-rect>> | <<circle()>> | <<ellipse()>> |  <<polygon()>> | <<path()>>
+	</pre>
+
 	When using this syntax
 	to define shapes,
 	the <dfn for="<basic-shape>" export>reference box</dfn> is defined
@@ -460,7 +465,7 @@ Supported Shapes</h3>
 				shape-functions/xywh-function-valid.html
 			</wpt>
 
-		<dt><dfn>rect()</dfn> =
+		<dt><dfn>rect()</dfn>
 		<dd>
 			Defines a rectangle
 			via insets from the top and left edges of the [=reference box=].

css-shapes-2/Overview.bs

@@ -125,6 +125,13 @@ Basic Shapes</h2>
 	<a href="https://www.w3.org/TR/css-shapes/#basic-shape-functions">level 1</a>
 	section.
 
+	This level extends the <<basic-shape>> functions
+	to include ''shape()'':
+
+	<pre class=prod>
+		<<basic-shape>> = <<basic-shape-rect>> | <<circle()>> | <<ellipse()>> |  <<polygon()>> | <<path()>> | <<shape()>>
+	</pre>
+
 <h3 id="supported-basic-shapes">
 Supported Shapes</h3>