(1)
Appeared to be not documented at all what is not very good.
Suggest page https://www.zabbix.com/documentation/2.4/manual/config/items/itemtypes/zabbix_agent

Suggested text:
In case of <mode> defined as "nowait" the key immediately returns just "1", disregarding of command execution results.

Zabbix agent 2.4.7rc1:

  7597:20151113:150504.035 listener #1 [processing request]
  7597:20151113:150504.035 Requested [system.run[sleep 3, nowait]]
  7597:20151113:150504.035 Executing command 'sleep 3'
  7597:20151113:150504.040 Sending back [1]
  7597:20151113:150504.040 listener #1 [waiting for connection]

(2)
Also, another thing. Looking at https://www.zabbix.com/documentation/2.4/manual/appendix/command_execution and reading first sentence I feel that something is wrong.
Is that correct that "remote commands" are included to the list ?

As I know they are executed used "nowait" flag, so it's wrong to include them to the list, IMO.
https://www.zabbix.com/documentation/2.4/manual/config/notifications/action/operation/remote_command