Fix incomplete at-docs block

[fredrikekre]

Include working doc strings instead of bailing completely in at-docs block.

Fix #799

[mortenpi]

Big 👍 to including the docstrings that are possible to include.

However, now there is nothing about the failed ones in the generated output, which I think makes them easy to miss (unless you do strict builds). So I would still output something for the failed docstrings too. Could we maybe just push a generated !!! warn adminition into the doctree? Or maybe keep a list with the DocsNodes and have the writer interpret it?

[fredrikekre]

also fixes #474

[fredrikekre]

Now results in:

from

```@docs
TestPackage.foo
TestPackage.baz
TestPackage.bar
```

when TestPackage.baz is missing.

[mortenpi]

This looks awesome!

Have you tested it with all the output formats, to make sure all handle the updated DocsNodes correctly? It might be good to have a test case in test/examples too (although, we might inadvertently already do).

[fredrikekre]

Have you tested it with all the output formats, to make sure all handle the updated DocsNodes correctly?

Yes.

It might be good to have a test case in test/examples too (although, we might inadvertently already do).

Exists at the top of https://github.com/JuliaDocs/Documenter.jl/blob/91fc140e7adef2eb17b85d8a2848e805877062ba/test/errors/src/index.md.