Elasticsearch version: all 2.X, 5.X, and 6.X releases

Plugins installed: NA

JVM version: NA

OS version: NA

Description of the problem including expected versus actual behavior:

The tasks.list and tasks.get REST API specs are missing the timeout parameter.

Steps to reproduce:

The timeout parameter is missing from the REST API spec for the tasks.list and tasks.get JSON files. Any client code or test validations that rely on these JSON files will be in error because of this missing param.

I've confirmed on ES 5.6.4 that the timeout param is accepted by the server:

> curl -XGET 'localhost:9200/_tasks?timeout=10s' | jq '.'
{
  "nodes": {
    "oXZa92DeSja4HL1sKHmsdw": { ... }
  }
}

Provide logs (if relevant): NA