Vague documentation of API

[droodman]

At https://juliastats.org/StatsModels.jl/stable/api/#StatsAPI.coefnames, I read:

[StatsAPI.coefnames](https://juliastats.org/StatsModels.jl/stable/api/#StatsAPI.coefnames)
—
Function
coefnames(model::StatisticalModel)

Return the names of the coefficients.

I have just learned through trial and error and debugging that the RegressionTables.jl package expects the return values to be strings, not symbols. It seems to me that the API should precisely specify the types that coefnames() and similar functions can return.