Improve wording about returned event

Improve the wording of the event that is returned by `queue::submit` and
friends.

Since the new wording says that the event represents the *command* that
is submitted to the queue, it seemed awkward for the _Effects_ paragraph
to say that a *command group function* is submitted to the queue.  I
replaced this with wording saying that the command group function is
immediately called, which is consistent with what we say in the
introductory paragraphs of section 4.6.5 "Queue class".  The new wording
also notes that the command group function may submit no more than one
command to the queue, which is consistent with the existing wording in
section 4.9.3 "Command group scope".

adoc/chapters/programming_interface.adoc

@@ -3225,11 +3225,12 @@ template <typename T>
 event submit(T cgf)
 ----
 
-_Effects:_ Submit a <<command-group-function-object>> to the queue, in order to
-be scheduled for execution on the device.
+_Effects:_ Immediately calls the <<command-group-function-object>> [code]#cgf#,
+which may submit no more than one <<command>> to the queue for execution on the
+device.
 
-_Returns:_ An event which the application can use to determine when the
-<<command>> associated with the <<command-group-function-object>> has completed.
+_Returns:_ An event which represents the <<command>> which is submitted to the
+queue.
 
 '''
 
@@ -3240,14 +3241,15 @@ template <typename T>
 event submit(T cgf, queue& secondaryQueue)
 ----
 
-_Effects:_ Submit a <<command-group-function-object>> to the queue, in order to
-be scheduled for execution on the device.
+_Effects:_ Immediately calls the <<command-group-function-object>> [code]#cgf#,
+which may submit no more than one <<command>> to the queue for execution on the
+device.
 On a kernel error, this <<command-group-function-object>> may be scheduled for
 execution on the secondary queue [code]#secondaryQueue# as described in
 <<sec::fallback-mechanism>>.
 
-_Returns:_ An event which the application can use to determine when the
-<<command>> associated with the <<command-group-function-object>> has completed.
+_Returns:_ An event which represents the <<command>> which is submitted to the
+queue.
 If the command is scheduled on [code]#secondaryQueue#, the event is associated
 with that queue.
 
@@ -3360,7 +3362,8 @@ _Effects (3):_ Equivalent to calling [api]#queue::submit# with a command group
 function that calls [code]#handler::depends_on(depEvents)# and
 [code]#handler::single_task(kernelFunc)#.
 
-_Returns:_ An event which represents the command that is submitted to the queue.
+_Returns:_ An event which represents the <<command>> which is submitted to the
+queue.
 
 '''
 
@@ -3412,7 +3415,8 @@ _Effects (6):_ Equivalent to calling [api]#queue::submit# with a command group
 function that calls [code]#handler::depends_on(depEvents)# and
 [code]#handler::parallel_for(executionRange, rest)#.
 
-_Returns:_ An event which represents the command that is submitted to the queue.
+_Returns:_ An event which represents the <<command>> which is submitted to the
+queue.
 
 '''
 
@@ -3438,7 +3442,8 @@ _Effects (3):_ Equivalent to calling [api]#queue::submit# with a command group
 function that calls [code]#handler::depends_on(depEvents)# and
 [code]#handler::memcpy(dest, src, numBytes)#.
 
-_Returns:_ An event which represents the command that is submitted to the queue.
+_Returns:_ An event which represents the <<command>> which is submitted to the
+queue.
 
 '''
 
@@ -3515,7 +3520,8 @@ _Effects (8):_ Equivalent to calling [api]#queue::submit# with a command group
 function that calls [code]#handler::require(src)#,
 [code]#handler::require(dest)#, and [code]#handler::copy(src, dest)#.
 
-_Returns:_ An event which represents the command that is submitted to the queue.
+_Returns:_ An event which represents the <<command>> which is submitted to the
+queue.
 
 '''
 
@@ -3541,7 +3547,8 @@ _Effects (3):_ Equivalent to calling [api]#queue::submit# with a command group
 function that calls [code]#handler::depends_on(depEvents)# and
 [code]#handler::memcpy(ptr, value, numBytes)#.
 
-_Returns:_ An event which represents the command that is submitted to the queue.
+_Returns:_ An event which represents the <<command>> which is submitted to the
+queue.
 
 '''
 
@@ -3578,7 +3585,8 @@ _Effects (4):_ Equivalent to calling [api]#queue::submit# with a command group
 function that calls [code]#handler::require(dest)# and
 [code]#handler::fill(dest, src)#.
 
-_Returns:_ An event which represents the command that is submitted to the queue.
+_Returns:_ An event which represents the <<command>> which is submitted to the
+queue.
 
 '''
 
@@ -3603,7 +3611,8 @@ _Effects (3):_ Equivalent to calling [api]#queue::submit# with a command group
 function that calls [code]#handler::depends_on(depEvents)# and
 [code]#handler::prefetch(ptr, numBytes)#.
 
-_Returns:_ An event which represents the command that is submitted to the queue.
+_Returns:_ An event which represents the <<command>> which is submitted to the
+queue.
 
 '''
 
@@ -3629,7 +3638,8 @@ _Effects (3):_ Equivalent to calling [api]#queue::submit# with a command group
 function that calls [code]#handler::depends_on(depEvents)# and
 [code]#handler::mem_advise(ptr, numBytes, advice)#.
 
-_Returns:_ An event which represents the command that is submitted to the queue.
+_Returns:_ An event which represents the <<command>> which is submitted to the
+queue.
 
 '''
 
@@ -3645,7 +3655,8 @@ _Effects:_ Equivalent to calling [api]#queue::submit# with a command group
 function that calls [code]#handler::require(acc)# and
 [code]#handler::update_host(acc)#.
 
-_Returns:_ An event which represents the command that is submitted to the queue.
+_Returns:_ An event which represents the <<command>> which is submitted to the
+queue.
 
 '''