Better error message when no doc found

Today we had this error in our CI pipeline that puzzled us for quite some time:

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

We eventually figured it out; but what would have helped us a lot would have been if there had been also a line number (or at least a rough range) within src/abelian/structural.md where the error was triggered.

In this case, the @ref was not in the .md file but rather was from a docstring. So it would have been extremely helpful to be told that, too: i.e. something like "from docstring of function Foo in file src/bar.jl:123" or whatever.