doc: add some info on `tty#setRawMode()`

Checklist

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

Affected core subsystem(s)

doc, tty

Description of change

Partially taken from https://linux.die.net/man/3/cfmakeraw

A very simple test script:

if (process.argv[2] === 'raw')
  process.stdin.setRawMode(true)

process.stdin.on('data', (chunk) => {
  console.log(chunk)
  console.log(chunk.toString())
})

Related to https://github.com/nodejs/node/pull/10037 Refs(?): http://docs.libuv.org/en/v1.x/tty.html#c.uv_tty_mode_t