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

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())
})
```

Refs: https://github.com/nodejs/node/pull/10037
PR-URL: https://github.com/nodejs/node/pull/10147
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/tty.md

@@ -50,6 +50,13 @@ raw device. Defaults to `false`.
 added: v0.7.7
 -->
 
+Allows configuration of `tty.ReadStream` so that it operates as a raw device.
+
+When in raw mode, input is always available character-by-character, not
+including modifiers. Additionally, all special processing of characters by the
+terminal is disabled, including echoing input characters.
+Note that `CTRL`+`C` will no longer cause a `SIGINT` when in this mode.
+
 * `mode` {boolean} If `true`, configures the `tty.ReadStream` to operate as a
   raw device. If `false`, configures the `tty.ReadStream` to operate in its
   default mode. The `readStream.isRaw` property will be set to the resulting