Issue #398: Make documentation consistent across models

[athowes]

Description

This PR closes #398.

• Removes documentation mentions of "A data.frame containing line list data." where it's not about the epidist_linelist_data class
• Trys to more generally describe the inputs to each function
• Moves a function in the naive_model.R position to be standard with latent_model.R

Note that I don't touch some mentions of "direct" in this PR which are handled by #446.

This PR raised some follow up issues which I have added (#448).

Checklist

• My PR is based on a package issue and I have explicitly linked it.
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• I have added or updated unit tests where necessary.
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