Merge pull request swiftlang#33737 from xwu/nominal-edu

[Docs] Revise wording for nominal types educational note

Author: xwu

userdocs/diagnostics/nominal-types.md

@@ -1,6 +1,9 @@
-# Nominal types
-In Swift, a type is considered a nominal type if it is named.  In other words, it has been defined by declaring the type somewhere in code. Examples of nominal types include classes, structs and enums, all of which must be declared before using them. Nominal types are an important concept in Swift because they can be extended, explicitly initialized using the `MyType()` syntax, and may conform to protocols.
+# Nominal Types
 
-In contrast, non-nominal types have none of these capabilities. A non-nominal type is any type which is not nominal. They are sometimes called "structural types" because they are usually 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`.
+In Swift, a type is considered a nominal type if it has been explicitly named by a declaration somewhere in code. Examples of nominal types include classes, structures and enumerations. Nominal types are an important concept in Swift because they may conform to protocols, be extended, and have values created using the initializer syntax `MyType()`.
 
-Whether the name of a protocol refers to a nominal or non-nominal type depends on where it appears in code. When used in a declaration or extension like `extension MyProtocol { … }`, `MyProtocol` refers to a protocol type, which is nominal. This means that it may be extended and conform to protocols. However, when written as the type of a constant or variable, `MyProtocol` instead refers to a non-nominal, existential type. As a result, code like `let value: MyProtocol = MyProtocol()` is not allowed because `MyProtocol` refers to a non-nominal type in this context and cannot be explicitly initialized.
+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()`.
+
+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_.