doc,test: args to `buffer.copy` can be Uint8Arrays

This was semi-overlooked in https://github.com/nodejs/node/pull/10236 because it always worked and didn’t need additional changes.

Ref: https://github.com/nodejs/node/pull/10236

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

buffer

CI: https://ci.nodejs.org/job/node-test-commit/8059/