Mentioned that {{StringContext}} may apply only to regular operation …

…and regular argument types explicitly.

index.bs

@@ -10277,10 +10277,11 @@ adhere to the context the string is used in.
 The [{{StringContext}}] extended attribute must [=takes an identifier|take an identifier=]. The [=identifier=]
 must be one of "<code>html</code>", "<code>script-url</code>" and "<code>script</code>".
 
-A type annotated with the [{{StringContext}}] extended attribute must not appear in a construct
-that is not a [=regular attribute=] or a [=regular operation=]. A type annotated with the [{{StringContext}}]
-extended attribute must not appear in a [=read only=] attribute. The construct that the type annotated with
-the [{{StringContext}}] extended attribute appears in is its <dfn>related construct</dfn>.
+[{{StringContext}}] extended attribute may only annotate a type of a [=regular attribute=] or
+a [=regular operation=] argument. A type annotated with the [{{StringContext}}]
+extended attribute must not appear in a [=read only=] attribute. The [=regular attribute=] or
+a [=regular operation=] argument that the type annotated with the [{{StringContext}}] extended
+attribute appears in is its <dfn>related construct</dfn>.
 
 A type that is not {{DOMString}} or {{USVString}} must not be [=extended attributes associated with|associated with=]
 the [{{StringContext}}] extended attribute.