Merge pull request #65297 from xlgao-zju/improve-kubectl-completion-help

Automatic merge from submit-queue (batch tested with PRs 65297, 67179, 67116, 67011, 66842). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

improve kubectl completion help

**What this PR does / why we need it**:
Add note that 'bash-completion' is required on Linux too.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Just like what we did in #64361

**Special notes for your reviewer**:
cc @neolit123

**Release note**:

```release-note
NONE
```

pkg/kubectl/cmd/completion.go

@@ -67,6 +67,8 @@ var (
 
 
 		# Installing bash completion on Linux
+		## If bash-completion is not installed on Linux, please install the 'bash-completion' package
+		## via your distribution's package manager.
 		## Load the kubectl completion code for bash into the current shell
 		    source <(kubectl completion bash)
 		## Write bash completion code to a file and source if from .bash_profile