Add discussion of concurrency back to the tour.

Reverts commit efa3e9a.

Author: amartini51

TSPL.docc/GuidedTour/GuidedTour.md

@@ -2257,6 +2257,124 @@ print(fridgeIsOpen)
   ```
 -->
 
+## Concurrency
+
+You use `async` to mark an asynchronous function.
+You mark an asynchronous operation,
+like a call to an asynchronous function,
+by writing `await`
+if you want to wait for it to complete.
+
+```swift
+func bake(_ food: String) async -> String {
+    // ... wait for food to bake ...
+    return food
+}
+func makeCookies() async -> String {
+    let cookies = await bake("cookies")
+    return cookies
+}
+```
+
+<!--
+  - test: `guided-tour`
+
+  ```swifttest
+  -> func bake(_ food: String) async -> String {
+         // ... wait for food to bake ...
+         return food
+     }
+  -> func makeCookies() async -> String {
+         let cookies = await bake("cookies")
+         return cookies
+     }
+  ```
+-->
+
+You can use `async`-`let` to start an asynchronous operation
+without waiting for it to complete,
+which lets the operations run at the same time.
+Because you still need to wait for the operation to finish
+before you can use the value it returns,
+you write `await` when you use that constant.
+
+```swift
+async let cookies = makeCookies()
+async let bread = bake("bread")
+
+let bakedGoods = await [cookies, bread]
+```
+
+<!--
+  - test: guided-tour-async
+
+  ```swifttest
+  -> async let cookies = makeCookies()
+  -> async let bread = bake("bread")
+  ---
+  -> let bakedGoods = await [cookies, bread]
+  ```
+-->
+
+Actors are similar to classes,
+except they ensure that different asynchronous functions
+can all interact with an instance of the same actor at the same time.
+
+```swift
+actor Oven {
+    var contents: [String] = []
+    func bake(_ food: String) -> String {
+        let index = contents.endIndex
+        contents.append(food)
+        // ... wait for food to bake ...
+        contents.remove(at: index)
+        return food
+    }
+}
+```
+
+<!--
+  - test: `guided-tour`
+
+  ```swifttest
+  -> actor Oven {
+         var contents: [String] = []
+         func bake(_ food: String) -> String {
+             let index = contents.endIndex
+             contents.append(food)
+             // ... wait for food to bake ...
+             contents.remove(at: index)
+             return food
+         }
+     }
+  ```
+-->
+
+When you call a method on an actor or access one of its properties,
+you mark that code with `await`
+to indicate that it might have to wait for other code
+that's already running on the actor to finish.
+
+```swift
+let oven = Oven()
+let biscuits = await oven.bake("biscuits")
+for item in await oven.contents {
+    print(item)
+}
+```
+
+<!--
+  - test: `guided-tour`
+
+  ```swifttest
+  -> let oven = Oven()
+  -> let biscuits = await oven.bake("biscuits")
+  -> for item in await oven.contents {
+         print(item)
+     }
+  ```
+-->
+
 ## Generics
 
 Write a name inside angle brackets