Mention timeboxing for estimation process

It came out of the discussion from gitlab-org/create-stage#12707 (comment 415785763). It'd be helpful for us to have a hard time limit to avoid feeling the urge to almost solve the issue and spending lots of time during estimation session.

It's not very productive in my opinion, because:

1. We delay estimating issues because we are not sure how long it'd take.
2. We had to switch context back and forth so it would have a negative impact on our deliverables so shortening this process would be helpful.
3. It's an estimation in the end regardless of how much time we spend on it unless we actually work on it. We need to accept the fact that our estimation is never going to be precise until the issue is closed.

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