Document interchangability with julia 0.4

I'm sure this leaves out important details, but at least it's a start.
`@docstrings` isn't actually needed any more if it's the 0-arity version. I probably should have documented that already.

`@doc` now automatically generates a module's `__METADATA__` if it's missing in a similar manner to the way `Base.@doc` works. See https://github.com/MichaelHatherly/Docile.jl/blob/063358da8f5de205b732753fd43444a404dc5842/src/macros.jl#L107. So we can leave out the `else` branch I'd think.