Update undefined reference warning and :skip_undefined_reference_warnings_on option

[wojtekmach]

Previously we were warning like this:

warning: documentation references function ExUnit.Case.__using__/1 but it doesn't exist or isn't public (parsing Kernel.use/2 docs)

Now, to be consistent with other Elixir warnings, we output:

warning: documentation references function ExUnit.Case.__using__/1 but it is undefined or private
  lib/elixir/lib/kernel.ex: Kernel.use/2

We're also making a breaking change to
:skip_undefined_reference_warnings_on, previously we could give it the
generated id of an extra e.g.: ["compatibility-and-deprecations"], now
we'd give it the path instead:
["lib/elixir/pages/compatibility-and-deprecations.md"].

[eksperimental]

I am getting a warning when generating Elixir docs.

warning: documentation references function ExUnit.Case.__using__/1 but it is undefined or private
  lib/elixir/lib/kernel.ex:4768: Kernel.use/2

ExUnit.Case.__using__/1 is neither private, nor undefined. It is a public macro with @doc false. Should we update the warning message?

[josevalim]

I believe this text is using the docs definition of private (i.e. @doc false is private) but we can probably clarify.

[eksperimental]

I believe this text is using the docs definition of private (i.e. @doc false is private) but we can probably clarify.

I can submit a PR

[wojtekmach]

I believe this text is using the docs definition of private

Yeah, that was the idea.