criu --help: rectify --join-ns

* Fix English and rephrase
* Mark with curly braces that PID and NS_FILE are exclusive options
* Fix 80 columns violations
* Remove usage examples
* Remove the "experimental feature" warning
* Add an empty line before "Check options" header

As for removals, I believe --help output is not the proper place for
examples or notices.

Was:
  -J|--join-ns NS:PID|NS_FILE[,EXTRA_OPTS]
                        Join exist namespace and restore process in it.
                        Namespace can be specified in pid or file path format.
                            --join-ns net:12345 or --join-ns net:/foo/bar.
                        Extra_opts is optional, for now only user namespace support:
                            --join-ns user:PID,UID,GID to specify uid and gid.
                        Please NOTE: join-ns with user-namespace is not fully tested.
                        It may be dangerous to use this feature
Check options:

Now:
  -J|--join-ns NS:{PID|NS_FILE}[,OPTIONS]
                        Join existing namespace and restore process in it.
                        Namespace can be specified as either pid or file path.
                        OPTIONS can be used to specify parameters for userns:
                            user:PID,UID,GID

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>