Need more info about automatic stats settings

[rytaft]

The automatic stats docs (https://www.cockroachlabs.com/docs/v19.1/cost-based-optimizer.html#controlling-automatic-statistics) mention two of the settings used to control automatic stats: sql.stats.automatic_collection.fraction_stale_rows and sql.stats.automatic_collection.min_stale_rows, but they don't give enough info to really understand how they work. One thing that would be helpful would be a direct link to those settings on the settings page, but it would also help to add a bit more info in the "Controlling automatic statistics" section. Some points we should emphasize:

• The target number of stale rows in a particular table before a refresh is actually equal to target_stale_rows = fraction_stale_rows * <size of table> + min_stale_rows.
• We don't necessarily perform a refresh when exactly that target number of rows are stale, because the decision to start a refresh is probabilistic. After each mutation operation (INSERT, UPDATE, or DELETE), the probability of a refresh is P(refresh) = (# rows updated, inserted, deleted)/target_stale_rows. The result is that on average, we will perform a refresh after target_stale_rows are stale, but it may be higher or lower.
• To ensure that we always have stats and they aren't too stale, we also always perform a refresh if there are no stats or if it's been a long time since the last refresh (a "long time" is defined as 2x the average time between refreshes over the last 4 refreshes).

I don't think we necessarily need all this detail, but I've put it here so you can judge what to include or not. Please let me know if you have any questions -- thanks!
cc @RaduBerinde