Xcode 7 beta 4 release notes mention it is now possible to add top-level documentation comments for throws:

Swift documentation comments recognize a top-level list item - Throws: ... which should be used to document what errors can be thrown and why. These appear alongside parameters and return descriptions in Xcode's QuickHelp. (21621679)

Here is an example function that is marked as throws:

/// This is a function that can throw an error
func someFunctionThatThrowsAnError(param: String) throws -> String {

To add the top-level throws item to Xcode QuickHelp, add a - Throws: ENTER ERROR DESCRIPTION HERE in a documentation comment above the function (The leading - before Throws: is required for this to work). Then option-click on the name of the function to bring up Xcode QuickHelp:

/// This is a function that can throw an error
/// - Throws: This will throw an error if `param.characters.count` is less than 3
func someFunctionThatThrowsAnError(param: String) throws -> String {

Throws documentation comment QuickHelp