Clarify std::env::args encoding requirements

[coyaSONG]

Closes #4796.

Clarifies that std::env::args panics when an operating-system argument cannot be represented as a Rust String. The text now calls out the platform-specific cases: invalid UTF-8 on Unix and invalid UTF-16 on Windows.

This avoids implying that Unicode itself is invalid while remaining accurate across platforms.

Tested with:

• npx --yes dprint check src/ch12-01-accepting-command-line-arguments.md
• mdbook build using the CI version, 0.5.1
• bash ci/validate.sh
• cargo run --bin lfp src

AI-assisted: Codex. I verified the wording against the official std::env::args and OsString documentation and reviewed the generated HTML.