update the local storage description from 'mocktikv' to 'unistore'

[tangenta]

What is changed, added or deleted? (Required)

In 'dev' and 'v5.0' version, the default value of --store TiDB command-line argument has been changed to 'unistore'.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

• This PR is translated from: update the local storage description from 'mocktikv' to 'unistore' docs-cn#5874
• Other reference link(s): *: support plain insert/point_get/table_read on cluster index table tidb#17674

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[TomShawn]

/label v5.0
/translation from-docs-cn
/label needs-cherry-pick-5.0
/size small

[TomShawn]

/label require-LGT1
/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 8e890d9

[ti-srebot]

cherry pick to release-5.0 in PR #5161