Update the DeviceLab README.md with updated warnings and info on how to run tests on a local machine #160242

Use case

Use case: Engineers get up-to-date information from the DeviceLab readme

The https://github.com/flutter/flutter/blob/master/dev/devicelab/README.md file explains how to run DeviceLab tests locally. But it omits some important details that differ from the usual behaviour of running tests engineers are familiar with:

1. By default, the failing tests are autoretried. It makes sense to mention this as it is likely any engineer running the tests locally does not want that behaviour and wants to disable it.
2. The DeviceLab test runner automatically reboots the Android and iOS devices it runs tests on. While this behaviour currently can't be disabled, it still makes sense to tell engineers about it, as otherwise, devices (seemingly) randomly restarting looks like a serious bug.

As an engineer, it would be very useful to have these things mentioned in the documentation.

Proposal

Proposal: Update the DeviceLab readme

Update the DeviceLab Readme at https://github.com/flutter/flutter/blob/master/dev/devicelab/README.md and add a warning about reboots and a small section about disabling the automatic retries for failed tests.