Explain potential performance overhead of events

adoc/extensions/sycl_khr_free_function_commands.adoc

@@ -4,6 +4,13 @@ This extension provides an alternative mechanism for submitting commands to a
 device via free-functions that require developers to opt-in to the creation of
 [code]#event# objects.
 
+The creation of [code]#event# objects may incur overheads that increase the
+latency of submitting commands to devices, even if the [code]#event# object is
+immediately discarded and never used.
+Requiring developers to opt-in to the creation of events is therefore expected
+to improve the performance of many SYCL programs, by ensuring that SYCL
+developers only pay the cost associated with using events when necessary.
+
 == Dependencies
 
 This extension has no dependencies on other extensions.
@@ -479,8 +486,8 @@ _Preconditions_:
   accessible on the device;
 * [code]#src# and [code]#dest# both point to allocations of at least
   [code]#count# elements of type [code]#T#; and
-* If either [code]#src# or [code]#dest# is a pointer to a USM allocation,
-  that allocation was created from the same context as the handler's queue.
+* If either [code]#src# or [code]#dest# is a pointer to a USM allocation, that
+  allocation was created from the same context as the handler's queue.
 
 _Effects (1)_: Equivalent to [code]#h.copy(src, dest, count)#.