docs(x/staking): update readme (backport #22216)

[mergify]

Description

Ref:
#21429

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Author Checklist

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I have...

• included the correct type prefix in the PR title, you can find examples of the prefixes below:

• confirmed ! in the type prefix if API or client breaking change

• targeted the correct branch (see PR Targeting)

• provided a link to the relevant issue or specification

• reviewed "Files changed" and left comments if necessary

• included the necessary unit and integration tests

• added a changelog entry to CHANGELOG.md

• updated the relevant documentation or specification, including comments for documenting Go code

• confirmed all CI checks have passed

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I have...

• confirmed the correct type prefix in the PR title
• confirmed all author checklist items have been addressed
• reviewed state machine logic, API design and naming, documentation is accurate, tests and test coverage

Summary by CodeRabbit

• Documentation

  • Enhanced clarity and accessibility of the staking module documentation.
  • Updated descriptions for state management and data structures.
  • Added a new validation function for the PowerReduction parameter.

• Bug Fixes

  • Improved validation checks to ensure parameters meet required conditions.

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This is an automatic backport of pull request #22216 done by [Mergify](https://mergify.com).