clarify that SVM is optional for all OpenCL 3.0 devices (#913)

Author: bashbaug

api/opencl_architecture.asciidoc

@@ -1051,8 +1051,9 @@ There are three types of SVM in OpenCL
               | No
 |====
 
-Coarse-Grained buffer SVM is required in the core OpenCL specification.
-The two finer grained approaches are optional features in OpenCL.
+Coarse-Grained buffer SVM is a required feature for OpenCL 2.0, 2.1, or 2.2
+devices and an optional feature for OpenCL 3.0 or newer devices.
+Fine-Grained SVM is an optional feature for all OpenCL devices.
 The various SVM mechanisms to access host memory from the work-items
 associated with a kernel instance are <<svm-summary-table, summarized
 above>>.