Add additional documentation about `@moduletag`s (#7397)
Add additional documentation about `@moduletag`s (#7397)

Using it before ExUnit.Case can lead to compilation errors.
elixir-lang/elixir,michalmuskala/elixir,lexmag/elixir,lexmag/elixir,kelvinst/elixir,ggcampinho/elixir,pedrosnk/elixir,ggcampinho/elixir,joshprice/elixir,gfvcastro/elixir,kelvinst/elixir,kimshrier/elixir,gfvcastro/elixir,pedrosnk/elixir,kimshrier/elixir