Update `Kernel.ParallelCompiler` docs reflecting hard-deprecation changes

[eksperimental]

Elixir and Erlang/OTP versions

Elixir v1.19-rc1

Current behavior

In the Kernel.ParallelCompiler we can read:

It returns {:ok, modules, warnings} or {:error, errors, warnings} by default but we recommend using return_diagnostics: true so it returns diagnostics as maps as well as a map of compilation information.

but the warning now says:

warning: you must pass return_diagnostics: true when invoking Kernel.ParallelCompiler

The former is a suggestion, the latter is a requirement.

Expected behavior

Warnings and docs be consistent