docs: clarify bilingual documentation policy in AGENTS.md

Updated language requirements section to explicitly document:
- Feature documentation: Required in both English and Japanese
- Detailed technical documentation: English required, Japanese optional
- Project-level documentation: Required in both languages

Added rationale explaining that detailed docs are primarily for developers
who typically read English technical documentation, and the significant
volume makes translation effort substantial with limited additional value.

Addresses Minor Observation #1 from PR review.

Co-authored-by: FT-FumihikoUnno <[email protected]>

Author: github-actions[bot]

AGENTS.md

@@ -113,9 +113,21 @@ Each ROS2 package in `src/` typically contains:
 ## Documentation Standards
 
 ### Language Requirements
-All documentation must be provided in **both English and Japanese**:
-- **English**: Primary documentation (e.g., `AGENTS.md`, `feature.md`)
-- **Japanese**: Translation with `-ja` suffix (e.g., `AGENTS-ja.md`, `feature-ja.md`)
+Documentation language requirements vary by type:
+
+**Feature Documentation** (`doc/feature/`):
+- **Required** in both English and Japanese
+- **English**: Primary documentation (e.g., `feature.md`)
+- **Japanese**: Translation with `-ja` suffix (e.g., `feature-ja.md`)
+
+**Detailed Technical Documentation** (`doc/detail/`):
+- **English**: Required for all detailed implementation documentation
+- **Japanese**: Optional (translations welcomed but not mandatory)
+- Rationale: Detailed technical documentation is primarily for developers who typically read English technical documentation. The significant volume of detailed docs makes translation effort substantial while providing limited additional value.
+
+**Project-level Documentation** (e.g., `AGENTS.md`, `README.md`):
+- **Required** in both English and Japanese
+- Follow the same `-ja` suffix convention
 
 ### File Naming Convention
 ```