I noticed the docs/Modules.rst is marked as being pre-Swift 1.0. Although it does have changes as through late 2016. Is that document up-to-date enough to rely on?
I've given it a quick skim just now and nothing in it stood out as egregiously wrong or misleading. It could definitely use some work though, several incomplete sections and some discussion of unimplemented bits like Swift-side submodules. If you wanted to tidy it up or expand on the text, patches would be welcome!
(It's also worth reading the Serialization.rst document, and familiarizing yourself with the linked clang module documentation, the term "module" is somewhat overloaded, and the machinery of each definition is somewhat intertwined.)