Add completion to help message (#478)

Fixes #477

docs/cmd/kn.md

@@ -22,6 +22,7 @@ Manage your Knative building blocks:
 
 ### SEE ALSO
 
+* [kn completion](kn_completion.md)	 - Output shell completion code
 * [kn plugin](kn_plugin.md)	 - Plugin command group
 * [kn revision](kn_revision.md)	 - Revision command group
 * [kn route](kn_route.md)	 - Route command group

docs/cmd/kn_completion.md

@@ -0,0 +1,47 @@
+## kn completion
+
+Output shell completion code
+
+### Synopsis
+
+
+This command prints shell completion code which needs to be evaluated
+to provide interactive completion
+
+Supported Shells:
+ - bash
+ - zsh
+
+```
+kn completion [SHELL] [flags]
+```
+
+### Examples
+
+```
+
+ # Generate completion code for bash
+ source <(kn completion bash)
+
+ # Generate completion code for zsh
+ source <(kn completion zsh)
+```
+
+### Options
+
+```
+  -h, --help   help for completion
+```
+
+### Options inherited from parent commands
+
+```
+      --config string       kn config file (default is $HOME/.kn/config.yaml)
+      --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --log-http            log http traffic
+```
+
+### SEE ALSO
+
+* [kn](kn.md)	 - Knative client
+

pkg/kn/commands/completion/completion.go

@@ -25,7 +25,7 @@ import (
 
 const (
 	desc = `
-This command prints shell completion code which need to be evaluated
+This command prints shell completion code which needs to be evaluated
 to provide interactive completion
 
 Supported Shells:
@@ -47,7 +47,6 @@ func NewCompletionCommand(p *commands.KnParams) *cobra.Command {
 		Long:      desc,
 		ValidArgs: []string{"bash", "zsh"},
 		Example:   eg,
-		Hidden:    true, // Don't show this in help listing
 		Run: func(cmd *cobra.Command, args []string) {
 			if len(args) == 1 {
 				switch args[0] {