[css-gcpm-3] Simplify syntax definitions (w3c#5632)

Author: lahmatiy

css-gcpm-3/Overview.bs

@@ -56,7 +56,7 @@ The 'string-set' property copies the text content of an element into a ''named s
 
 <pre class="propdef">
 Name: <dfn id="string-set">string-set</dfn>
-Value: [[ <<custom-ident>> <<content-list>>] [, <<custom-ident>> <<content-list>>]* ] | none
+Value: [ <<custom-ident>> <<content-list>> ]# | none
 Initial: none
 Applies to: all elements, but not pseudo-elements
 Inherited: no
@@ -161,7 +161,7 @@ The value of the “header” string will be “Loomings”.
 The ''string()'' function is used to copy the value of a named string to the document, via the 'content' property. This function requires one argument, the name of the named string. Since the value of a named string may change several times on a page (as new instances of the element defining the string appear) an optional second argument indicates which value of the named string should be used.
 
 <pre class="prod">
-	<dfn>string()</dfn> = string( <<custom-ident>> [ , [ first | start | last | first-except] ]? )
+	<dfn>string()</dfn> = string( <<custom-ident>> , [ first | start | last | first-except ]? )
 </pre>
 
 
@@ -366,7 +366,7 @@ Just as with ''string()'', ''element()'' takes an optional keyword to describe w
 </pre>
 
 <pre class="prod">
-	<dfn>element()</dfn> = element( <<custom-ident>> [ , [ first | start | last | first-except] ]? )
+	<dfn>element()</dfn> = element( <<custom-ident>> , [ first | start | last | first-except ]? )
 </pre>
 
 <h2 id="footnotes">