Docstring check confused with reexported function

[nalimilan]

At https://github.com/JuliaData/CategoricalArrays.jl/pull/367/checks?check_run_id=3575057492 I see this error:

[ Info: CheckDocument: running document checks.
┌ Error: 1 docstring not included in the manual:
│ 
│     Missings.levels :: Tuple{CategoricalArray}
│ 
│ 
│ These are docstrings in the checked modules (configured with the modules keyword)
│ that are not included in @docs or @autodocs blocks.
└ @ Documenter.DocChecks ~/.julia/packages/Documenter/ruzxx/src/DocChecks.jl:69
[ Info: Populate: populating indices.
ERROR: LoadError: `makedocs` encountered an error. Terminating build

Yet the docstring is included in the manual, but it's DataAPI.levels (where the function is originally defined) rather than Missings.levels.

I could fix it by changing the definition of levels in Missings (see JuliaData/Missings.jl#138) from

export levels
import DataAPI
const levels = DataAPI.levels

to:

export levels
using DataAPI: levels

That's not a big deal but I figured I would report it in case the fix would be easy.