Ex_doc: Deal with custom link that links to undefined/hidden module

[eksperimental]

`Code.Typespec` 

[code typespec](`Code.Typespec`)

Code.Typespec is a hidden module, in the first example, and ExDoc what it does is to treat format the text inside the backticks as such generating <code class="inline">Code.Typespec</code>

but when using a custom link, the generated code is
<a href="`Code.Typespec`">code typespec</a>

I am not too sure what is the best way to deal with this, probably removing the backticks and linking to Code.Typespec,
but having the backticks does not seem right

[josevalim]

Great find. We should warn and remove the link IMO. :)

[eksperimental]

We are currently warning.
I am not 100% convinced of removing the link because it may hide the error, even though we are warning. A project may generate multiple warnings and this one can get buried there.
But removing it will be consistent with how we deal in the first example

[wojtekmach]

Good find. I think we should definitely remove the link because there's nowhere to link to.

[eksperimental]

Alright +1 with that as we don't have any other options

[eksperimental]

I will take care of this.