Clarify wording of default-constructed accessors

Avoid using the phrase "the underlying memory" for a default
constructed accessor because this implies that there is some
underlying memory.

adoc/chapters/programming_interface.adoc

@@ -6672,7 +6672,7 @@ accessor()
   * The return values of [code]#get_pointer()# and [code]#get_multi_ptr()# are
     unspecified.
   * A default constructed accessor can be passed to a <<sycl-kernel-function>>,
-    but attempting to access the underlying memory produces undefined behavior.
+    but attempting to access data elements from it produces undefined behavior.
 --
 
 a@
@@ -8194,7 +8194,7 @@ local_accessor()
   * The return values of [code]#get_pointer()# and [code]#get_multi_ptr()# are
     unspecified.
   * A default constructed local accessor can be passed to a
-    <<sycl-kernel-function>>, but attempting to access the underlying memory
+    <<sycl-kernel-function>>, but attempting to access data elements from it
     produces undefined behavior.
 --