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
PR-URL: https://github.com/nodejs/node/pull/11486


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>