The swift man page on Linux and macOS is essentially the same, providing only a starting point. Running 'swift -help' gives a listing of options, but leaves out much detail, which is no surprise.
But I've been looking for some hours and still haven't found anything explaining the options in depth. In fact, running grep on the source tree from the top level reveals that the only place where options are mentioned are in the source file ToolChains.cpp, which is what parses the options.
It seems like they are almost entirely undocumented.
Unfortunately, the -help output is pretty much the only documentation for many options currently. The good news is we're beginning to put more thought into how the situation could be improved. If there are any specific enhancements you'd like to see I'd encourage you to file bugs at bugs.swift.org (confusing or missing documentation is considered a bug).
In the meantime, feel free to ask here on the forums about anything that's unclear, there should be plenty of people who can help.