Update compatibility chapter for Swift 6 (#308)

Fixes: rdar://127062330

Author: amartini51

TSPL.docc/GuidedTour/Compatibility.md

@@ -2,15 +2,24 @@
 
 Learn what functionality is available in older language modes.
 
-This book describes Swift 5.10,
-the default version of Swift that's included in Xcode 15.3.
-You can use Xcode 15.3 to build targets
-that are written in either 5.10, Swift 4.2, or Swift 4.
-
-When you use Xcode 15.3 to build Swift 4 and Swift 4.2 code,
-most Swift 5.10 functionality is available.
+This book describes Swift 6,
+the default version of Swift that's included in Xcode XXX.
+You can use the Swift 6 compiler to build code
+that's written in Swift 6, Swift 5, Swift 4.2, or Swift 4.
+
+When you use the Swift 6 compiler
+to build code that uses the Swift 5 language mode,
+you can use the new features from Swift 6 ---
+they're enabled either by default or by an upcoming feature flag.
+However, to enable strict concurrency checking,
+you need to upgrade to the Swift 6 language mode.
+
+In addition,
+when you use Xcode 15.3 to build Swift 4 and Swift 4.2 code,
+most Swift 5 functionality is still available.
 That said,
-the following changes are available only to code that uses 5.10 or later:
+the following changes are available only to code
+that uses the Swift 5 language mode:
 
 - Functions that return an opaque type require the Swift 5.1 runtime.
 - The `try?` expression doesn't introduce an extra level of optionality
@@ -20,18 +29,18 @@ the following changes are available only to code that uses 5.10 or later:
   For example, `UInt64(0xffff_ffff_ffff_ffff)` evaluates to the correct value
   rather than overflowing.
 
-Concurrency requires 5.10 or later,
+Concurrency requires the Swift 5 language mode
 and a version of the Swift standard library
 that provides the corresponding concurrency types.
 On Apple platforms, set a deployment target
 of at least iOS 13, macOS 10.15, tvOS 13, or watchOS 6.
 
-A target written in 5.10 can depend on
-a target that's written in Swift 4.2 or Swift 4,
+A target written in Swift 6 can depend on
+a target that's written in Swift 5, Swift 4.2 or Swift 4,
 and vice versa.
 This means, if you have a large project
 that's divided into multiple frameworks,
-you can migrate your code from Swift 4 to 5.10
+you can migrate your code to a newer language version
 one framework at a time.
 
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