Fix up command-buffer enqueue error after PR-1382

KhronosGroup#1382 added support
for deferring setting arguments on a kernel command which is updatable.
To achieve this we added an extra command-buffer state, "Finalized"
which is entered when a command-buffer has been finalized but doesn't
yet have all it's arguments set.

However, if a user tries to enqueue a command-buffer in this state
it shouldn't be valid. Therefore update our current wording about
when an command-buffer can be enqueued to say the specifically the executable
state, rather than just after the finalization operation, which could
result in the command-buffer being in either the finalization or executable
state.

Author: EwanC

api/opencl_runtime_layer.asciidoc

@@ -15135,7 +15135,8 @@ execution was successfully queued, or one of the errors below:
 
   * {CL_INVALID_COMMAND_BUFFER_KHR} if _command_buffer_ is not a valid
     command-buffer.
-  * {CL_INVALID_OPERATION} if _command_buffer_ has not been finalized.
+  * {CL_INVALID_OPERATION} if _command_buffer_ is not in the
+    {CL_COMMAND_BUFFER_STATE_EXECUTABLE_KHR} state.
   * {CL_INVALID_VALUE} if _queues_ is `NULL` and _num_queues_ is > 0, or
     _queues_ is not `NULL` and _num_queues_ is 0.
   * {CL_INVALID_VALUE} if _num_queues_ is > 0 and not the same value as