Clarify version vs mode (#45)

* Clarify version vs mode #43

* Update Guide.docc/MigrationGuide.md

Co-authored-by: Holly Borla <[email protected]>

---------

Co-authored-by: Holly Borla <[email protected]>

Author: mattmassicotte

Guide.docc/MigrationGuide.md

@@ -23,11 +23,11 @@ Adopting the Swift 6 language mode is entirely under your control
 on a per-target basis.
 Targets that build with previous modes, as well as code in other
 languages exposed to Swift, can all interoperate with
-modules that have been migrated to Swift 6.
+modules that have been migrated to the Swift 6 language mode.
 
 It is possible you have been incrementally adopting concurrency features
 as they were introduced.
-Or, you may have been waiting for Swift 6 to begin using them.
+Or, you may have been waiting for the Swift 6 release to begin using them.
 Regardless of where your project is in this process, this guide provides
 concepts and practical help to ease the migration.
 
@@ -39,8 +39,13 @@ Here you will find articles and code examples that will:
 - Provide techniques for incremental adoption.
 - Present strategies to resolve common problems.
 
-> Important: The Swift 6 language mode is opt-in.
+> Important: The Swift 6 language mode is _opt-in_.
 Existing projects will not switch to this mode without configuration changes.
+>
+> There is an important distinction between the _compiler version_
+and _language mode_.
+The Swift 6 compiler supports four distinct language modes: "6", "5", "4.2",
+and "4".
 
 ## Topics