Make rt=NULL settings explicit

[jamesmbaazam]

Description

This PR closes #761.

This PR makes explicit the settings that are applied when rt=NULL in functions that use that argument.

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]

This was what I had in mind when I raised this issue. Happy to rephrase based on reviews but I keep going back into the code to remind myself of the applied settings, so I thought would be useful to expose it.

[sbfnk]

Good idea