docs: add disadvantages section to accepting-strings idiom

[husamettinarabaci]

📄 Description

This PR adds content to the previously empty "Disadvantages" section of the accepting-strings idiom.

The section outlines the potential trade-offs and caveats when working with &CStr in FFI contexts, including:

• Lifetime management complexity
• Null-termination requirements
• Fallibility of UTF-8 conversions
• Potential thread safety concerns with raw pointers

These additions aim to give developers a clearer understanding of the limitations and risks, complementing the advantages already described.

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🔍 Motivation

The "Disadvantages" section was previously marked as None?. This contribution completes that section with relevant, up-to-date notes based on current Rust FFI practices (as of Rust 1.77+).

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✅ Checklist

• One idiom file changed only (accepting-strings.md)
• Markdown formatting validated
• Commit message follows docs: convention
• No semantic meaning changed in existing content