Pareto diagnostic defaults and doc updates

[n-kall]

Summary

After discussion with @avehtari, this PR:

• exposes individual Pareto diagnostic functions (min_ss, khat_threshold, convergence_rate) (see #322)
• changes the default for return_k to FALSE in pareto_smooth. (#325)
• Improves and simplifies the messages provided by the Pareto diagnostics.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[github-actions[bot]]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 40af2e35967620cf7426fa66cc228b2d851ba1f7 is merged into master:

•   :ballot_box_with_check:as_draws_array: 101ms -> 101ms [-0.73%, +0.69%]
•   :rocket:as_draws_df: 36.3ms -> 34.2ms [-7.39%, -3.89%]
•   :rocket:as_draws_list: 198ms -> 193ms [-4.38%, -0.33%]
• :exclamation::snail:as_draws_matrix: 30.1ms -> 33ms [+7.56%, +11.55%]
•   :ballot_box_with_check:as_draws_rvars: 165ms -> 164ms [-2.51%, +1.04%]
•   :ballot_box_with_check:summarise_draws_100_variables: 721ms -> 723ms [-0.42%, +1.02%]
•   :ballot_box_with_check:summarise_draws_10_variables: 80.3ms -> 80.2ms [-1.13%, +0.73%] Further explanation regarding interpretation and methodology can be found in the documentation.

[n-kall]

If no other comments, I think this can be merged