Document CBS in CHANGES and using.rst.

CHANGES

@@ -17,6 +17,11 @@ Notable User Facing Changes
 - Added a user env POCL_BITCODE_FINALIZER that can be used to
   call a custom script that manipulates the final bitcode before
   passing it to the code generation.
+- Pulled in alternative work-group function mode for non-SPMD from hipSYCL:
+  Continuation-based synchronization is slightly more general than POCL's
+  current kernel compiler, but allows for fewer hand-rolled optimizations.
+  CBS is expected to work for kernels that POCL's current kernel compiler
+  does not support.
 
 3.1 December 2022
 =================

doc/sphinx/source/using.rst

@@ -342,6 +342,12 @@ pocl.
               might avoid storing work-item context to memory.
               However, the code bloat is increased with larger
               WG sizes.
+    
+    cbs    -- Use continuation-based synchronization to execute work-items
+              on non-SPMD devices.
+              CBS is expected to work for kernels that 'loops' does not support.
+              For most other kernels it is expected to perform slightly worse.
+              Also enables the LLVM LoopVectorizer.
 
 - **POCL_WORK_GROUP_SPECIALIZATION**