docs(module:dropdown): additional notes on nzMode of nz-menu

[stygian-desolator]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: /~https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

close #6756

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #6773 (ec5e66a) into master (1ec3f64) will decrease coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6773      +/-   ##
==========================================
- Coverage   89.88%   89.70%   -0.19%     
==========================================
  Files         479      484       +5     
  Lines       15489    15665     +176     
  Branches     2522     2553      +31     
==========================================
+ Hits        13923    14053     +130     
- Misses        943      974      +31     
- Partials      623      638      +15     

Impacted Files	Coverage Δ
components/steps/steps.component.ts	91.13% <0.00%> (-5.79%)	⬇️
components/popconfirm/popconfirm.ts	96.00% <0.00%> (-4.00%)	⬇️
components/steps/step.component.ts	95.91% <0.00%> (-1.81%)	⬇️
components/table/src/table-style.service.ts	94.82% <0.00%> (-1.73%)	⬇️
components/tabs/tab-nav-bar.component.ts	82.33% <0.00%> (-0.71%)	⬇️
components/core/config/config.ts	100.00% <0.00%> (ø)
components/steps/steps.module.ts	100.00% <0.00%> (ø)
components/popconfirm/popconfirm.module.ts	100.00% <0.00%> (ø)
components/image/image-preview.component.ts	90.13% <0.00%> (ø)
components/core/services/image-preload.ts	30.55% <0.00%> (ø)
... and 10 more

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[F-JJTH]

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