unify docs to use variable instead of variables

[avehtari]

Summary

Closes #429

Copyright and Licensing

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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]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if ebb53ec is merged into master:

• ✔️as_draws_array: 134ms -> 134ms [-0.64%, +0.57%]
• ✔️as_draws_df: 67.9ms -> 68.4ms [-0.49%, +1.98%]
• ✔️as_draws_list: 156ms -> 156ms [-0.55%, +0.61%]
• ✔️as_draws_matrix: 28.4ms -> 28.4ms [-0.6%, +0.57%]
• ✔️as_draws_rvars: 75.6ms -> 76.1ms [-0.08%, +1.37%]
• ✔️summarise_draws_100_variables: 721ms -> 720ms [-0.26%, +0.03%]
• 🚀summarise_draws_10_variables: 83.2ms -> 83ms [-0.51%, -0.02%]
  Further explanation regarding interpretation and methodology can be found in the documentation.

[paul-buerkner]

So you changed the name in the doc but the argument name in the function remained the same? If so, this should lead to an R CMD check failure du to code-documentionat mismatch. Or am I misreading something?

[avehtari]

I think this is what we did discuss, but I guess I did misunderstood. How would you solve this?

[paul-buerkner]

Either rename the argument names too (breaks backwards compatibility) or leave things as is. The latter would mean that using "variable" still works due to R's partial matching of argument names. The former would mean that only "variable" would work but "variables" would no longer work. Not sure if that is desirable. I am sorry I haven't been clear with this previously. @florence-bockting perhaps you have some other ideas?