Add an example and cross reference to TSPL.

Fixes <rdar://79689446>

Author: amartini51

stdlib/public/core/KeyPath.swift

@@ -200,6 +200,14 @@ public class PartialKeyPath<Root>: AnyKeyPath { }
 internal enum KeyPathKind { case readOnly, value, reference }
 
 /// A key path from a specific root type to a specific resulting value type.
+///
+/// The most common way to make an instance of this type
+/// is by using a key-path expression like `\SomeClass.someProperty`.
+/// For more information,
+/// see [Key-Path Expressions[keypath] in *[The Swift Programming Language][tspl]*.
+///
+/// [keypath]: https://docs.swift.org/swift-book/ReferenceManual/Expressions.html#ID563
+/// [tspl]: https://docs.swift.org/swift-book/
 public class KeyPath<Root, Value>: PartialKeyPath<Root> {
   @usableFromInline
   internal final override class var _rootAndValueType: (