Clarify the relation between the secure context definitions.

Author: Ms2ger

index.bs

@@ -10351,7 +10351,9 @@ on any other construct.
 
 The [{{SecureContext}}] extended attribute must [=takes no arguments|take no arguments=].
 
-By default, constructs are <dfn export>available in both secure and non-secure contexts</dfn>.
+A construct is <dfn export>available in both secure and non-secure contexts</dfn> if it is not
+[=available only in secure contexts=] (i.e., if no [{{SecureContext}}] extended attribute applies
+to it).
 
 <div algorithm>