[C++ for OpenCL] Improve description of restricted C++ features. (#582)

Added the following restrictions that were omitted:
- references to functions;
- pointers to member functions;

Added clarification for:
- thread-safety of static locals initialization.

Added references to C++ spec sections where applicable.

Author: AnastasiaStulova

cxx4opencl/diff2cxx.txt

@@ -12,8 +12,26 @@ however, there are some differences that are documented in this section.
 
 The following {cpp} language features are not supported:
 
-  * Virtual functions
-  * Exceptions
-  * `dynamic_cast` operator
-  * Non-placement `new`/`delete` operators
-  * Standard C++ libraries
+  * Virtual functions ({cpp}17 `[class.virtual]`);
+  * References to functions including member functions ({cpp}17 `[class.mfct]`);
+  * Pointers to class member functions (in addition to the regular non-member
+    functions that are already restricted in OpenCL C);
+  * Exceptions ({cpp}17 `[except]`);
+  * `dynamic_cast` operator ({cpp}17 `[expr.dynamic.cast]`);
+  * Non-placement `new`/`delete` operators ({cpp}17
+    `[expr.new]`/`[expr.delete]`);
+  * Standard C++ libraries ({cpp}17 `[library]`).
+
+:fn-feature-macro: footnote:[The macro belongs to the list of {cpp}20's \
+feature test macros.]
+
+Simultaneous initialization of static local objects performed by
+different work-items is not guaranteed to be free from race-conditions.
+Whether an implementation provides such a guarantee is indicated by the
+presence of the `__cpp_threadsafe_static_init` feature test
+macro{fn-feature-macro}.
+
+The list above only contains extra restrictions that are not detailed in OpenCL
+C. As OpenCL restricts a number of C features, the same restrictions are
+inherited by C++ for OpenCL. The detailed list of C feature restrictions
+is provided in `OpenCL C v2.0 section 6.9`.