fix `@doc`  for abstract types

Documenting abstract types now works:
```julia
@doc "Doc abstract type" ->
abstract C
Docs.doc(C)
# ->   Doc abstract type
```
I tried to add a  test for it but did not manage to compare two markdown strings.  How would I do that? `md"a"==md"a"` returns false.
Yep. I'll also push the test-case (in a couple of minutes)