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

• 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.
• I have updated the documentation if required and rebuilt docs if yes (using devtools::document()).
• I have followed the established coding standards (and checked using lintr::lint_package()).
• 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.

[jamesmbaazam]

LGTM.

This all seems to come down to advising users to explore different values and not rely on the default values.