docs: fix the admin transfer description

[dannyward630]

Closes #305

Summary

• Correct the cancellation guidance: only the current admin can cancel a pending proposal.
• Document the same two-step admin-transfer flow for both ngo-registry and donation-vault.
• Clarify who can propose, accept, and cancel, and that the current admin retains control until acceptance.

Validation

• git diff --check passed.
• cargo build --workspace --target wasm32v1-none --release passed.
• cargo fmt --all -- --check reports pre-existing formatting differences in unmodified donation-vault files.
• cargo clippy --workspace --all-targets -- -D warnings and cargo test --workspace are blocked by pre-existing errors in unmodified contract code/tests: duplicate imports in ngo-registry tests and missing donation-vault client methods referenced by tests.