Change to using typed docstrings.

An image?  An audio note?  An MS word file?  ;-)   Of course, everything in Julia has a text/plain representation, but that's not the same as saying it is a string.

More importantly, using `Docstring` forces any documentation to descend from a single abstract class, and forces it to depend on Docile.  For example, it prevents you from using an `ASCIIString` for documentation, which seems odd!   Also, it inverts the dependency graph: it means that a Markdown package would have to depend on Docile, rather than the other way around, in order to define a `Markdown` type that is usable in documentation.