Clarify that default context contains root devices

adoc/chapters/programming_interface.adoc

@@ -970,7 +970,7 @@ The [code]#platform# class also provides constructors, but constructing a new
 the objects returned by [api]#platform::get_platforms#.
 
 Each platform has an associated default context which contains all of the
-devices in the platform.
+<<root-device,root devices>> in the platform.
 This default context does not have an asynchronous error handler.
 Applications can retrieve a copy of this default `context` object, for example,
 by constructing a `queue`.