docs(iam): document UpdateUserUsername

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.47059% with 8 lines in your changes missing coverage. Please review.

Project coverage is 70.86%. Comparing base (5bf53e0) to head (b5cb443).
Report is 392 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/namespaces/iam/v1alpha1/iam_cli.go	76.47%	7 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4359      +/-   ##
==========================================
- Coverage   75.45%   70.86%   -4.60%     
==========================================
  Files         202      294      +92     
  Lines       44323    65520   +21197     
==========================================
+ Hits        33444    46429   +12985     
- Misses       9653    17460    +7807     
- Partials     1226     1631     +405     

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