doc: clarify out-of-bounds behavior of buf[index]

PR-URL: https://github.com/nodejs/node/pull/11286
Fixes: https://github.com/nodejs/node/issues/11244


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ron Korving <ron@ronkorving.nl>