add label manpage and fix help

Signed-off-by: Ma Shimiao <mashimiao.fnst@cn.fujitsu.com>

Backported to v1.0.0.rc1 from 5de50e5 opencontainers#189 (cherry-pick applied
cleanly).

Signed-off-by: W. Trevor King <wking@tremily.us>

cmd/ocitools/generate.go

@@ -36,7 +36,7 @@ var generateFlags = []cli.Flag{
 	cli.StringFlag{Name: "mount-label", Usage: "selinux mount context label"},
 	cli.StringSliceFlag{Name: "tmpfs", Usage: "mount tmpfs"},
 	cli.StringSliceFlag{Name: "args", Usage: "command to run in the container"},
-	cli.StringSliceFlag{Name: "env", Usage: "add environment variable"},
+	cli.StringSliceFlag{Name: "env", Usage: "add environment variable e.g. key=value"},
 	cli.StringFlag{Name: "cgroups-path", Usage: "specify the path to the cgroups"},
 	cli.StringFlag{Name: "mount-cgroups", Value: "no", Usage: "mount cgroups (rw,ro,no)"},
 	cli.StringSliceFlag{Name: "bind", Usage: "bind mount directories src:dest:(rw,ro)"},

man/ocitools-generate.1.md

@@ -58,10 +58,10 @@ read the configuration from `config.json`.
   Current working directory for the process
 
 **--env**=[]
-  Set environment variables
-  This option allows you to specify arbitrary
-environment variables that are available for the process that will be launched
-inside of the container.
+  Set environment variables e.g. key=value.
+  This option allows you to specify arbitrary environment variables
+  that are available for the process that will be launched inside of
+  the container.
 
 **--gid**=GID
   Gid for the process inside of container
@@ -84,6 +84,9 @@ inside of the container.
   The special *PATH* `host` removes any existing IPC namespace from the
   configuration.
 
+**--label**=[]
+  Add annotations to the configuration e.g. key=value.
+
 **--mount**=*PATH*
   Use a mount namespace where *PATH* is an existing mount namespace file
   to join. The special *PATH*  empty-string  creates a new namespace.