Add daemon documentation on user namespaces feature

Remove the experimental docs for user namespaces and add similar content
to the `docker daemon` command documentation.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

man/docker-daemon.8.md

@@ -53,6 +53,7 @@ docker-daemon - Enable daemon mode
 [**--tlskey**[=*~/.docker/key.pem*]]
 [**--tlsverify**]
 [**--userland-proxy**[=*true*]]
+[**--userns-remap**[=*default*]]
 
 # DESCRIPTION
 **docker** has two distinct functions. It is used for starting the Docker
@@ -223,6 +224,9 @@ unix://[/path/to/socket] to use.
 **--userland-proxy**=*true*|*false*
     Rely on a userland proxy implementation for inter-container and outside-to-container loopback communications. Default is true.
 
+**--userns-remap**=*default*|*uid:gid*|*user:group*|*user*|*uid*
+    Enable user namespaces for containers on the daemon. Specifying "default" will cause a new user and group to be created to handle UID and GID range remapping for the user namespace mappings used for contained processes. Specifying a user (or uid) and optionally a group (or gid) will cause the daemon to lookup the user and group's subordinate ID ranges for use as the user namespace mappings for contained processes.
+
 # STORAGE DRIVER OPTIONS
 
 Docker uses storage backends (known as "graphdrivers" in the Docker