Bring alpha_sd docs up to date

[zsusswein]

Description

This PR closes #861.

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.

This is a docs-only change so I didn't re-run code checks.

After the initial Pull Request

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

[jamesmbaazam]

Thanks @zsusswein. LGTM.