TiDB auto increment MySQL-compatible behavior clarification (#19861)

[ti-chi-bot]

This is an automated cherry-pick of #19861

What is changed, added or deleted? (Required)

Improved the MySQL compatibility mode section in auto-increment.md by:

1. Providing a clearer introduction that immediately states the purpose and benefits
2. Adding practical examples showing behavior differences between AUTO_ID_CACHE 1 and default mode
3. Explaining when and why gaps might occur in the sequence
4. Restructuring the version-specific notes for better readability
5. Adding information about v8.1.0 changes regarding forceRebase operation

These changes make it easier for users to:

• Understand the practical differences between MySQL compatibility mode and default mode
• See concrete examples of ID allocation behavior in different scenarios
• Understand the performance implications across different TiDB versions

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

Since this change includes information about behavior changes across multiple versions (v6.4.0 and v8.1.0) and documents current functionality, it should be applied to:

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

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

• This PR is translated from: TiDB auto increment MySQL-compatible behavior clarification docs#20152, Update auto increment by adding explanations docs#20390
• Other reference link(s):

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

[lilin90]

/approve

[ti-chi-bot]

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