SetShardTabletControl: Improve flag help output

[mattlord]

Description

When attempting to use this command to clear up errant topo data, it was realized that the help output for the flag was unclear and misleading. I've tried to make the help text clear based on following the code to see what the flags actually do.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

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• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

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Backward compatibility

• Protobuf changes should be wire-compatible.
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• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[deepthi]

We do need to update website docs, but we can do that separately.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.67%. Comparing base (3d36adb) to head (1ff6aba).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16376   +/-   ##
=======================================
  Coverage   68.66%   68.67%           
=======================================
  Files        1548     1548           
  Lines      199083   199083           
=======================================
+ Hits       136694   136711   +17     
+ Misses      62389    62372   -17     

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[rohit-nayak-ps]

❤️

We should probably break this out into explicit commands, one for denied tables and another for query service: I will create an issue to track this.