[C++ for OpenCL] Add section w/ kernel function restrictions. (#583)

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

Author: AnastasiaStulova

CXX_for_OpenCL.txt

@@ -46,6 +46,10 @@ include::cxx4opencl/diff2openclc.txt[]
 
 include::cxx4opencl/address_spaces.txt[]
 
+<<<<
+
+include::cxx4opencl/kernel.txt[]
+
 <<<
 
 include::cxx4opencl/references.txt[]

cxx4opencl/kernel.txt

@@ -0,0 +1,28 @@
+// Copyright 2021 The Khronos Group. This work is licensed under a
+// Creative Commons Attribution 4.0 International License; see
+// http://creativecommons.org/licenses/by/4.0/
+
+[[kernel_function]]
+=== Kernel functions
+
+Kernel functions have implicit C linkage ({cpp}17 `[dcl.link]`) which
+means that {cpp} specific features are not supported. Therefore, the kernel
+functions:
+
+  * Can not be class members ({cpp}17 `[class.mfct]`);
+  * Can not be overloaded ({cpp}17 `[over]`);
+  * Can not be function templates ({cpp}17 `[temp.fct]`).
+
+:fn-ker-par-ref: footnote:[Whether C++ features (e.g references) can be used \
+in functions with C linkage is implementation-defined ({cpp}17 [dcl.link\]).]
+
+Moreover the types used in parameters of the kernel functions must be:
+
+  * Trivial and standard-layout types {cpp}17 `[basic.types]` (plain old data
+    types) for parameters passed by value;
+  * Standard layout types for pointer parameters. The same applies to
+    references{fn-ker-par-ref} if an implementation supports them in kernel
+    parameters.
+
+These are additional restrictions to the list detailed in `OpenCL C v2.0
+section 6.9`.