docs: Remove subjective and exaggerated expressions from documentation

[takeshishimada]

Summary

This PR removes subjective and exaggerated expressions from the documentation (README and DocC files) to maintain a more objective and professional tone.

Changes Made

Replaced subjective expressions with objective language:

• ❌ "full support for" → ✅ "supports"
• ❌ "Thread-safety guaranteed" → ✅ "Provides thread-safety"
• ❌ "making it easy to track" → ✅ "allowing you to track"
• ❌ "State scope is clear" → ✅ "State scope matches the view lifecycle"
• ❌ "No need to manage global state" → ✅ "Reduces the need to manage global state"
• ❌ "can be freely defined" → ✅ "can be defined"
• ❌ "making dependencies easy to track" → ✅ "allowing dependencies to be tracked"
• ❌ "data races caught at compile time" → ✅ "compile-time data race detection"
• ❌ "Observe all actions" → ✅ "Observe actions"

Files Modified

• README.md
• Sources/Flow/Flow.docc/Flow.md
• Sources/Flow/Flow.docc/CoreConcepts.md

Rationale

The original expressions like "guaranteed", "full support", and "easy" are subjective marketing terms that can be perceived as exaggerated claims. This change improves documentation quality by:

• Using factual, verifiable language
• Maintaining professional technical writing standards
• Avoiding absolute claims that may not hold in all edge cases
• Aligning with the more measured tone of the Japanese documentation

Test Plan

• Documentation builds successfully
• All technical information remains accurate
• Tone is consistent across all documentation files

🤖 Generated with Claude Code