restore alpha prior SD to 0.05

[sbfnk]

Description

This PR closes #834.

Still happy to review/reconsider if other values might work better.

I think I've got the squares / square roots right (and fixed them in the old news item) but may have got confused.

Initial submission checklist

• [X] My PR is based on a package issue and I have explicitly linked it.
• [ ] I have tested my changes locally (using devtools::test() and devtools::check()).
• [ ] I have added or updated unit tests where necessary.
• [X] I have updated the documentation if required and rebuilt docs if yes (using devtools::document()).
• [X] I have followed the established coding standards (and checked using lintr::lint_package()).
• [X] I have added a news item linked to this PR.

After the initial Pull Request

• [ ] I have reviewed Checks for this PR and addressed any issues as far as I am able.