doc: describe API ERR_INVALID_PROTOCOL context

ERR_INVALID_PROTOCOL docs should describe the API it relates to.

See: https://github.com/nodejs/node/pull/27390/files#r278246361

PR-URL: https://github.com/nodejs/node/pull/27393


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>