doc: buffer.fill() can zero-fill on invalid input

Note that buffer.fill() can zero-fill on some input types if no
valid fill data remains, but does nothing on other input types.

PR-URL: https://github.com/nodejs-private/node-private/pull/120
Fixes: https://github.com/nodejs-private/security/issues/193
Refs: https://github.com/nodejs-private/node-private/pull/118


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>