Document UndoManagerRegistration Further

Author: thecoolwinter

CodeEdit/Features/Editor/Models/Restoration/UndoManagerRegistration.swift

@@ -10,11 +10,19 @@ import CodeEditTextView
 
 /// Very simple class for registering undo manager for files for a project session. This does not do any saving, it
 /// just stores the undo managers in memory and retrieves them as necessary for files.
+///
+/// Undo stacks aren't stored on `CEWorkspaceFile` or `CodeFileDocument` because:
+/// - `CEWorkspaceFile` can be refreshed and reloaded at any point.
+/// - `CodeFileDocument` is released once there are no editors viewing it.
+/// Undo stacks need to be retained for the duration of a workspace session, enduring editor closes..
 final class UndoManagerRegistration: ObservableObject {
     private var managerMap: [CEWorkspaceFile.ID: CEUndoManager] = [:]
 
     init() { }
-
+    
+    /// Find or create a new undo manager.
+    /// - Parameter file: The file to create for.
+    /// - Returns: The undo manager for the given file.
     func manager(forFile file: CEWorkspaceFile) -> CEUndoManager {
         if let manager = managerMap[file.id] {
             return manager