doc: describe SNI host name format

Ref: https://github.com/nodejs/node/pull/23329

PR-URL: https://github.com/nodejs/node/pull/23357
Refs: https://github.com/nodejs/node/pull/23329


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>