[Core] Update documentation on ray.get on exception behavior

[dentiny]

Issue proposes to have better documentation on the timing to raise an exception when any task fails.

Closes #47231

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[jjyao]

What we want to document is the intended behavior that if you ray.get() multiple objects, if one object is an exception, ray.get() will immediately return without waiting for the remaining ones to finish. You can test it out by

import ray
import time

ray.init()

@ray.remote
def task1():
  raise ValueError("fail")

@ray.remote
def task2():
  time.sleep(1000000)


ray.get([task1.remote(), task2.remote()])

The PR you referred is actually a bug since it doesn't conform to the intended behavior.

[dentiny]

Ah gotcha, sorry I thought it's asking to document known issues part. Updated comments, thanks for the example!