fix: clarify synchronization licensing and documentation

[tisonkun]

Summary

• apply the standard ASF source header to the independently developed and substantially modified Mutex implementation while retaining Tokio copyright, MIT terms, and a commit-pinned source URL
• record the concrete Mutex and RwLock adaptations in LICENSE, while keeping the RwLock files under their existing third-party-derived header treatment
• revise public synchronization documentation around observable behavior, cancellation, ownership, and guard projection instead of internal semaphore, permit, waiter, and waker mechanics
• keep guard variance checks as doctest-only compile-fail coverage without exposing implementation-oriented sections on the public API pages
• standardize trivial Display implementations and empty vector construction for consistency

Validation

• cargo x test
• cargo test -p asyncband --all-features --doc
• cargo +nightly clippy --tests --all-features --all-targets --workspace -- -D warnings
• cargo +nightly fmt --all --check
• RUSTDOCFLAGS="-D warnings --cfg docsrs" cargo +nightly doc --workspace --all-features --no-deps
• hawkeye check
• typos asyncband/src

cargo x lint completed Clippy and rustfmt, then the local Taplo binary crashed in the macOS system-configuration library while running its format check.