Add glossary reference: "core SYCL specification"

adoc/chapters/architecture.adoc

@@ -239,8 +239,7 @@ In this case, the implementation may choose to document how the backend APIs
 correlate to SYCL and provide interoperation APIs that allow a SYCL application
 to make calls to both the SYCL APIs and the backend APIs.
 However, applications that make use of this ability are no longer conformant to
-the core SYCL specification and may not be portable to other implementations of
-SYCL.
+the <<core-spec>> and may not be portable to other implementations of SYCL.
 
 In order to aid with this sort of backend interoperation, SYCL provides the
 [code]#backend# enumeration, and most SYCL objects provide a member function