[NFC][SYCL][Graph][Doc] Clarify aspect relationship (#17996)

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

Author: EwanC

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