Allow to add more information in `is_flaky`
# What does this PR do?

As mentioned once offline, I think it's better for us to make a bit more effort to describe the situation for tests decorated by `is_flaky`. We don't always know the exact reasons (and for known cases, we don't always have good way to fix - at least not in a few months sometimes). A `description` is always good IMO.

For future PRs, if new tests being decorated with `is_flaky`, let's keep 👀 on if `description` is provided - despite it's an optional parameter 🙏 .
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