Update Mix.Task.preferred_cli_env/1 docs

[wojtekmach]

ExDoc main emitted the following warning on Elixir main:

    warning: documentation references function "Mix.CLI.preferred_cli_env/1" but it is hidden
    │
  1 │ defmodule Mix.Task do
    │ ~~~~~~~~~~~~~~~~~~~~~
    │
    └─ (mix 1.17.0-dev) lib/mix/lib/mix/task.ex:1: Mix.Task.preferred_cli_env/1

and the warning was pointing to the wrong location. Not sure if it's worth addressing but here's what happened: defdelegate without explicit @doc adds the @doc "See <delegating_to>" however here we're delegating to a module with @moduledoc false hence the warning.

I decided to just update the docs. At some point I imagine this function would become a @doc false anyway.

Before:

After:

[josevalim]

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