Improved the documentation regarding scheduler plugins (fixes #8719)

[RaphaelRobidas]

Closes #8719

• [ ] Tests passed (not all tests pass on the upstream HEAD)
• [x] Passes pre-commit run --all-files

[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[bot]]

Unit Test Results

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

    29 files  ±0      29 suites  ±0   11h 27m 20s :stopwatch: - 2m 27s  4 064 tests ±0   3 961 :white_check_mark: +1     97 :zzz: ±0  6 :x:  - 1  55 981 runs  ±0  53 811 :white_check_mark: +3  2 163 :zzz: ±0  7 :x:  - 3 

For more details on these failures, see this check.

Results for commit b87992e4. ± Comparison against base commit 33a281ff.