Better "no docs found" error message

Whenever I get the error message that reads

Error: no doc found for reference '[`rms`](@ref)' in src/validation.md.

I can never remember if the problem is

1. There is no docstring in the source code for this reference.
2. There is no docstring included in the docs for this reference.
3. Number 2 as a consequence of number 1.
4. Number 2 as a consequence of rms not being exported.

Would it be possible to somehow be more precise in these error messages? For example, I was just debugging why I got the message above, and it turned out that

1. I did have rms in a @docs block.
2. rms did have a docstring in the source
3. rms was not exported.

It took me quite a while to figure out that the problem was number 3 here, and it's not the first time :sweat_smile: