Document "should I stop" task checkpoints

[CaroFG]

taskCancellation type tasks in Meilisearch are prioritized, but it's important to understand that a task will only be effectively canceled when it reaches a specific checkpoint in its execution where it evaluates whether it should stop. These checkpoints are distributed at various points in the task's code, and sometimes there can be a significant interval before a task reaches the next checkpoint to evaluate the cancellation request.

Source: slack discussion with @Kerollmops