https://docs.inductiveautomation.com/display/DOC80/Expression+Overview+and+Syntax would be the obvious place to include some overview of what [.] and [~] mean, but they aren’t there. As far as I can tell, they aren’t explicitly explained in the documentation anywhere.
They are documented here:
Look at Using Relative Paths.
That said, the examples could be a bit more verbose.
Hope that helps.