[Docs] Fix RST formatting issues in diagonal, diff, and diagflat API docs

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Summary

This PR fixes RST formatting compliance issues found in three API documentation files under docs/api/paddle/.

Changes

docs/api/paddle/diagonal_cn.rst

• Fixed Chinese colon (:) to hyphen (-) after parameter type in **x** description
• Fixed full-width parentheses (int,可选) to regular parentheses (int,可选) for offset, axis1, axis2 parameters
• Fixed typo: 既主对角线 → 即主对角线 ("即" means "i.e./namely"; "既" is incorrect here)
• Fixed single backtick `Tensor` to double backtick ``Tensor`` in return value description
• Fixed comma separator to hyphen in return value description to match standard format

docs/api/paddle/diff_cn.rst

• Fixed single backticks `n` and `Tensor` to double backticks ``n`` and ``Tensor`` for proper RST inline code formatting (single backtick in RST creates a cross-reference, not inline code)
• Added space before 的 after n for readability
• Removed extraneous Chinese colon (:) from 代码示例: section title to match standard format (代码示例)

docs/api/paddle/diagflat_cn.rst

• Fixed single backtick `Tensor` to double backtick ``Tensor`` for proper RST inline code formatting

Compliance Standards Applied

Per the repository documentation standards in .github/copilot-instructions.md:

• RST inline code should use double backticks (``code``) not single backticks
• Parameter descriptions should use regular (ASCII) parentheses and hyphens, not Chinese/full-width equivalents
• Chinese text should use correct Chinese characters (typo fix: 既→即)

Generated by API Docs Checker

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🔄 Synced from https://github.com/StatefulDust/paddle-docs-agent/pull/8

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• docs/api/paddle/diagflat_cn.rst: 点击预览
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Pull request overview

该 PR 针对 docs/api/paddle/ 下的 3 个中文 API .rst 文档进行格式合规修正，主要聚焦于 RST inline code 标记、参数/返回值描述的标点与排版一致性，以减少 Sphinx/RST 构建告警并提升文档可读性。

Changes:

• 统一并修正 RST inline code 标记（如将 Tensor/n 调整为 Tensor/n）。
• 修正参数列表中的中文/全角标点与分隔符格式（如将中文冒号改为连字符、全角括号改为半角括号）。
• 修正小范围中文用字错误，并规范“代码示例”小节标题格式。

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
docs/api/paddle/diff_cn.rst	修正 n/Tensor 的 inline code 标记与“代码示例”标题格式，避免 RST 解释为交叉引用。
docs/api/paddle/diagonal_cn.rst	规范参数列表分隔符/括号格式，修正“既→即”，并统一返回值类型的 inline code 标记。
docs/api/paddle/diagflat_cn.rst	将参数描述中的 Tensor 修正为 Tensor 以符合 RST inline code 语法。