Improve deprecation messages

[menelaos]

Before submitting your PR, check that you've:

• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddocks for new, public APIs

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[menelaos]

I assume this PR falls under "documentation change" and thus does not need a new version.

[parsonsmatt]

Looks great, thank you!

[menelaos]

Thanks for the quick review!
Out of interest: what does the deprecation cycle look like for yesod? Is there a set period of time after which the primed versions will be renamed?

[parsonsmatt]

I don't think there's any official policy. I'd guess if there's been a Stackage LTS release with the deprecation notice then it's probably fine to do the rename.