document __init__

This fixes #8923.   @vtjnash, @JeffBezanson, please look over the text to make sure that it is accurate.

cc: @timholy 
If you want, you can fix such issues yourself, even without mucking with the git command line, because github provides a [Web UI](https://github.com/JuliaLang/julia/edit/master/doc/manual/modules.rst) to create pull requests if you click the pencil button above a file.