Exec remote API documentation

Includes
1. minor code cleanup 
2. addition of a TODO
3. updated CLI documentation.

Ping @cpuguy83 
what does `active` mean? i presume that you mean 'not stopped' but can I run it in a paused container? 

If it means a 'running' container only - can we use that? I didn't find any use of the word 'active' in our terminology :)