doc: clarify that the ctx argument is optional

Clarify that the `ctx` argument of the `SNICallback` callback is
optional.

Fixes: https://github.com/nodejs/node/issues/34085

PR-URL: https://github.com/nodejs/node/pull/34097


Reviewed-By: Ujjwal Sharma <ryzokuken@disroot.org>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>