fix(module: tree): fix disabled node status & doc

[simplejason]

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?

[x] 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

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #1737 into master will decrease coverage by <.01%.
The diff coverage is 92.3%.

@@            Coverage Diff             @@
##           master    #1737      +/-   ##
==========================================
- Coverage   96.05%   96.04%   -0.01%     
==========================================
  Files         466      466              
  Lines       11146    11151       +5     
  Branches     1456     1459       +3     
==========================================
+ Hits        10706    10710       +4     
  Misses        127      127              
- Partials      313      314       +1

Impacted Files	Coverage Δ
components/tree/nz-tree.service.ts	95.19% <92.3%> (-0.38%)	⬇️

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