[C++ for OpenCL] Added references to execution and memory models (#511)

Improved introduction by highlighting the fundamental difference in the execution and memory model between OpenCL and C/C++.

Co-authored-by: Sven van Haastregt <sven.vanhaastregt@arm.com>

Author: AnastasiaStulova

cxx4opencl/intro.txt

@@ -7,7 +7,12 @@
 
 This language is built on top of OpenCL C v2.0 and {cpp}17 enabling
 most of regular {cpp} features in OpenCL kernel code. Most functionality
-from OpenCL C and {cpp} is inherited.
+from {cpp} and OpenCL C is inherited. Since both OpenCL C and {cpp} are
+derived from C and moreover {cpp} is almost fully backward compatible with C,
+the main design principle of {cpp} for OpenCL is to reapply existing OpenCL
+concepts to {cpp}. Therefore, it is important to refer to OpenCL v2.0 section
+3.2 and section 3.3 detailing fundamental differences of OpenCL execution and
+memory models from the conventional C and {cpp} view.
 
 This document describes the programming language in details. It is not
 structured as a standalone document, but rather as an addition to `OpenCL C