refactor: document extra methods

[N3kox]

What type of PR is this?

• refactor schema.Document builtin extra methods
  • rename few method names and extra key
  • add sparse vector getter & setter（basically for indexer and retrieve）

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• This PR title match the format: <type>(optional scope): <description>
• The description of this PR title is user-oriented and clear enough for others to understand.
• Attach the PR updating the user documentation if the current PR requires user awareness at the usage level. User docs repo

(Optional) Translate the PR title into Chinese.

• 调整 schema.Document extra 的部分内置方法
  • 修改部分方法名称与 key
  • 新增 sparse vector getter setter，供 indexer & retriever 使用

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