doc: avoid referring to C array size

The size of arrays is measured in bytes in C, not in the number of
elements. The napi_get_cb_info function takes the length of the
array, that is, the number of elements it can contain, and not its size.

PR-URL: https://github.com/nodejs/node/pull/35300


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>