Add a reference to TSPL

Author: xwu

userdocs/diagnostics/nominal-types.md

@@ -4,4 +4,6 @@ In Swift, a type is considered a nominal type if it has been explicitly named by
 
 In contrast, non-nominal types do not have these capabilities. Many are obtained by composing other types. Examples include function types like `(Int) -> (String)`, tuple types like `(Int, String)`, metatypes like `Int.Type`, and special types like `Any` and `AnyObject`.
 
-Since a protocol is named by a declaration in code, it may conform to (in other words, refine) other protocols and it may be extended. However, when written as the type of a constant or variable such as `let value: MyProtocol`, the name refers to a distinct, non-nominal existential type that provides a "box" for a value of any concrete type that conforms to the protocol. The existential type itself does not conform to any protocols and cannot be extended, and a value cannot be created using the initializer syntax `MyProtocol()`.
+Since a protocol is named by a declaration in code, it may conform to (in other words, refine) other protocols and it may be extended. However, when written as the type of a constant or variable such as `let value: MyProtocol`, the name refers to a distinct, non-nominal existential type that provides a "box" for a value of any concrete type that conforms to the protocol. The existential type itself does not conform to any protocols and cannot be extended, and a value cannot be created using the initializer syntax `MyProtocol()`.
+
+For more on using existential types, see [Protocols as Types](https://docs.swift.org/swift-book/LanguageGuide/Protocols.html#ID275) in _The Swift Programming Language_.