Migrate docs for dashboard arguments

[shu-mutou]

Also, cleanup descriptions for dashboard --help.

e.g. remove description for default value like "Default: ~" to prevent duplicating with default value printed out by pflag.

Related to #4008

[maciaszczykm]

/lgtm
/approve

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[codecov]

Codecov Report

Merging #4164 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4164   +/-   ##
=======================================
  Coverage   46.19%   46.19%           
=======================================
  Files         194      194           
  Lines        8999     8999           
  Branches       83       83           
=======================================
  Hits         4157     4157           
  Misses       4586     4586           
  Partials      256      256

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