Add no-op note to command_barrier

adoc/extensions/sycl_khr_free_function_commands.adoc

@@ -766,6 +766,10 @@ Any commands submitted after this barrier cannot begin execution until:
 * All commands associated with this command's dependencies (e.g., via
   [code]#handler::depends_on#) have completed.
 
+{note}If a [code]#command_barrier# is submitted to an in-order queue and has no
+other dependencies (e.g., specified by [code]#handler::depends_on#), then this
+operation is a no-op.{endnote}
+
 '''
 
 .[apititle]#event_barrier#