Split up ReadWriteAttribute.

Fixes #398.

Author: Ms2ger

index.bs

@@ -1105,6 +1105,7 @@ The <dfn>qualified name</dfn> of an [=interface=] |interface| is defined as foll
         ReadWriteAttribute
         ReadWriteMaplike
         ReadWriteSetlike
+        InheritAttribute
 </pre>
 
 <pre class="grammar" id="prod-Inheritance">
@@ -1988,10 +1989,14 @@ are applicable only to regular attributes:
 
 <pre class="grammar" id="prod-ReadWriteAttribute">
     ReadWriteAttribute :
-        "inherit" AttributeRest
         AttributeRest
 </pre>
 
+<pre class="grammar" id="prod-InheritAttribute">
+    InheritAttribute :
+        "inherit" AttributeRest
+</pre>
+
 <pre class="grammar" id="prod-AttributeRest">
     AttributeRest :
         "attribute" TypeWithExtendedAttributes AttributeName ";"