Allow "constructor" as an argument name (#786)

Author: saschanaz

index.bs

@@ -685,6 +685,7 @@ underscore.
         "attribute"
         "callback"
         "const"
+        "constructor"
         "deleter"
         "dictionary"
         "enum"
@@ -714,7 +715,7 @@ keyword token is used, then the [=identifier=] of the operation argument
 is simply that token.
 
 The [=identifier=] of any of the abovementioned
-IDL constructs must not be "<code>constructor</code>",
+IDL constructs (except operation arguments) must not be "<code>constructor</code>",
 "<code>toString</code>",
 or begin with a <span class="char">U+005F LOW LINE ("_")</span> character.  These
 are known as <dfn id="dfn-reserved-identifier" export>reserved identifiers</dfn>.