Add additional information to DeviceLab readme for running DeviceLab tests locally (flutter#160243)

Fixes flutter#160242

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

Author: harri35

dev/devicelab/README.md

@@ -56,9 +56,12 @@ You can find where your Android SDK is using `flutter doctor -v`.
 
 ### Warnings
 
-Running the devicelab will do things to your environment.
+Running DeviceLab tests locally will do things to your environment.
 
-Notably, it will start and stop Gradle, for instance.
+Notably:
+
+- It will automatically start and stop Gradle on your machine
+- It will automatically reboot your target Android or iOS device after a certain amount of tests before running any additional tests on it. See the `checkForRebootRequired()` in `flutter/dev/devicelab/lib/framework/framework.dart` and `device.reboot()` in `flutter/dev/devicelab/lib/framework/devices.dart` for more.
 
 ### Running tests in `test/...`
 
@@ -82,6 +85,14 @@ To run multiple tests, repeat option `-t` (`--task`) multiple times:
 ../../bin/cache/dart-sdk/bin/dart bin/run.dart -t test1 -t test2 -t test3
 ```
 
+### Running tests without automatic retries
+
+By default, DeviceLab tests have an automatic retry logic built in. Any failing tests will be retried 2 additional times. This can be skipped by specifying the `--exit` option:
+
+```sh
+../../bin/cache/dart-sdk/bin/dart bin/test_runner.dart test --exit -t {NAME_OF_TEST}
+```
+
 ### Running tests against a local engine build
 
 To run device lab tests against a local engine build, pass the appropriate