Add docker-exec to main manpage

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)

docs/man/docker-exec.md

@@ -2,7 +2,7 @@
 % Docker Community
 % SEPT 2014
 # NAME
-docker-exec - Run a command in an existing container
+docker-exec - Run a command in an active container
 
 # SYNOPSIS
 **docker exec**

docs/man/docker.1.md

@@ -98,6 +98,9 @@ unix://[/path/to/socket] to use.
 **docker-events(1)**
   Get real time events from the server
 
+**docker-exec(1)**
+  Run a command in an active container
+
 **docker-export(1)**
   Stream the contents of a container as a tar archive