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.

Fixes: https://github.com/nodejs/node/issues/16491 Fixes: https://github.com/nodejs/node/issues/17405

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc