Clarify ThreadPoolExecutor#kill documentation

[oiahoon]

Summary

• clarify that executor #kill may interrupt in-progress tasks
• keep the existing queued-task and new-task guarantees explicit
• add the required changelog entry

Closes #1072

Verification

• bundle exec rspec spec/concurrent/executor/fixed_thread_pool_spec.rb spec/concurrent/executor/simple_executor_service_spec.rb (110 examples, 0 failures)
• deterministic FixedThreadPool#kill runtime check
• BUNDLE_WITH=documentation bundle exec rake yard:master
• bundle exec rake (2,795/2,796 examples passed; the timing-sensitive channel integration example that failed once passed on focused rerun)
• git diff --check