doc: fix entry for `napi_create_external_buffer`

Remove text regarding copying, because `napi_create_external_buffer`
does not copy.

Fixes: https://github.com/nodejs/node/issues/33471

PR-URL: https://github.com/nodejs/node/pull/34125


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Mathias Buus <mathiasbuus@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>