Clarify access::target::host_task

An accessor with access::target::host_task is accessible on
the host from within the host task lambda. It is necessary
to specify that no get_native_mem method is needed and also
that target::host_task means not only 'accessible within host_task'
but 'accessible within host_task on the host'. This is necessary
as host_task defines a scope, but it doesn't define where the
memory is accessible from.

adoc/chapters/programming_interface.adoc

@@ -6537,6 +6537,8 @@ capture the [code]#accessor# in a <<host-task>> can only use the accessor for
 interoperability through the [code]#interop_handle#, any other uses result in
 undefined behavior.
 
+An accessor with access target [code]#target::host_task# is accessible on the
+host, within the host task function for which the accessor is declared.
 Programs which specify the access target as [code]#target::host_task# and then
 use the [code]#accessor# from a <<sycl-kernel-function>> result in undefined
 behavior.