[SYCL][Graph] Clarify aspect relationship

SYCL-Graph contains two aspects for reporting extension support.
Make the relationship between the two aspects clearer to readers,
as it wasn't immediately obvious to reviewers of a
[application PR](ggml-org/llama.cpp#12873)
using these aspects.

Author: Ewan Crawford

sycl/doc/extensions/experimental/sycl_ext_oneapi_graph.asciidoc

@@ -321,6 +321,9 @@ devices with full graph support are more prevalent.
 
 |===
 
+A device which reports `aspect::ext_oneapi_graph` must also report
+`aspect::ext_oneapi_limited_graph`, as `aspect::ext_oneapi_graph` is a
+superset of the limited aspect.
 
 === Node