The create() API exposed RC4-40 and RC4-128 behind a doc comment, so an app
could write files whose encryption protects nothing. create() now returns
Unsupported for both schemes; create_legacy() and
Document::set_encryption_legacy() are the explicit ways to write a file that
must stay readable by an old reader. Tests and the encrypt example, which
read and write every algorithm, use the legacy entry points.