Chore: add information to cli command about vela cluster join without namespace creation

[ieuD]

Description of your changes

🤖 Generated by Copilot at 1f035e8

Summary

🐛📝🚀

Fix unwanted namespace creation in multicluster join. Update flag description for CreateNamespace in cluster.go.

CreateNamespace flag
clarified for multicluster
no more spring cleaning

Walkthrough

• Clarify the flag description for CreateNamespace in cluster join command (link)

Fixes #
Additional information for vela cluster join command which informs if with empty string in --create-namespace it won't create namespace in target cluster.
I have:

• Read and followed KubeVela's contribution process.
• Related Docs updated properly. In a new feature or configuration option, an update to the documentation is necessary.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Special notes for your reviewer

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (73c9c75) 65.57% compared to head (1f035e8) 65.58%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6367      +/-   ##
==========================================
+ Coverage   65.57%   65.58%   +0.01%     
==========================================
  Files         183      183              
  Lines       23885    23885              
==========================================
+ Hits        15662    15665       +3     
+ Misses       6643     6642       -1     
+ Partials     1580     1578       -2     

Flag	Coverage Δ
core-unittests	55.38% <ø> (+0.03%)	⬆️
e2e-multicluster-test	31.15% <ø> (+0.03%)	⬆️

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see 5 files with indirect coverage changes

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