Merge pull request KhronosGroup#831 from Pennycook/clarification/thread-safety

Clarify thread-safety guarantees

Author: gmlueck

adoc/chapters/programming_interface.adoc

@@ -11,8 +11,8 @@ available for any <<backend, SYCL backends>>, with the exception of the
 interoperability interface, which is described in general terms in this
 document, not pertaining to any particular <<backend>>.
 
-SYCL guarantees that all the member functions and special member functions of
-the SYCL classes described are thread safe.
+All functions defined in this specification are thread-safe, unless otherwise
+specified.
 
 The underlying types for all enumerations defined in this specification are
 implementation-defined.