doc: remove IPC channel implementation details

IPC messages are more complicated than a simple pipe passing JSON
objects separated by new line. This removes inaccurate notes about
implementation from the documentation.

PR-URL: https://github.com/nodejs/node/pull/17460
Fixes: https://github.com/nodejs/node/issues/16491
Fixes: https://github.com/nodejs/node/issues/17405


Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>