clarified concurrent.futures section in client.rst

[mercyo12]

Closes #2463

• Tests added / passed
• Passes pre-commit run --all-files

made the change suggested by @TomAugspurger here.

[GPUtester]

Can one of the admins verify this patch?

Admins can comment ok to test to allow this one PR to run or add to allowlist to allow all future PRs from the same author to run.

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

       20 files  ±       0         20 suites  ±0   10h 6m 20s ⏱️ - 1h 36m 32s
  3 747 tests +       6    3 627 ✔️ +       1     108 💤 +  2  12 ❌ +3 
34 968 runs   - 1 216  33 244 ✔️  - 1 173  1 705 💤  - 45  19 ❌ +2 

For more details on these failures, see this check.

Results for commit b5e7850. ± Comparison against base commit d6758bd.

This pull request removes 1 and adds 7 tests. Note that renamed tests count towards both.

distributed.deploy.tests.test_adaptive ‑ test_target_duration

distributed.cli.tests.test_dask_worker.test_listen_address_ipv6[tcp:..[ ‑ 1]:---nanny]
distributed.cli.tests.test_dask_worker.test_listen_address_ipv6[tcp:..[ ‑ 1]:---no-nanny]
distributed.deploy.tests.test_adaptive ‑ test_respect_average_nthreads
distributed.deploy.tests.test_adaptive ‑ test_scale_up_large_tasks[1]
distributed.deploy.tests.test_adaptive ‑ test_scale_up_large_tasks[inf]
distributed.deploy.tests.test_adaptive ‑ test_target_duration[1]
distributed.deploy.tests.test_adaptive ‑ test_target_duration[5]

♻️ This comment has been updated with latest results.

[mercyo12]

@fjetter hi, are there any more changes that need to be made?

[hendrikmakait]

@mercyo12: Thanks for your contribution!