Show example of nested protocols [SE-0404] (#235)

Fixes: rdar://117101695

Author: amartini51

TSPL.docc/LanguageGuide/NestedTypes.md

@@ -3,10 +3,11 @@
 Define types inside the scope of another type.
 
 Enumerations are often created to support a specific class or structure's functionality.
-Similarly, it can be convenient to define utility classes and structures
-purely for use within the context of a more complex type.
+Similarly, it can be convenient to define utility structures
+purely for use within the context of a more complex type,
+and protocols that are normally used in conjunction with a specific type.
 To accomplish this, Swift enables you to define *nested types*,
-whereby you nest supporting enumerations, classes, and structures
+whereby you nest supporting types like enumerations, structures, and protocols
 within the definition of the type they support.
 
 To nest a type within another type,