Add explanation for LambdaContext not being Sendable

Author: aryan-25

Sources/AWSLambdaRuntimeCore/Documentation.docc/Proposals/0001-v2-api.md

@@ -208,7 +208,11 @@ public protocol LambdaResponseWriter: ~Copyable {
 `ByteBufferAllocator`) will be removed.
 
 A new function `addBackgroundTask(_:)` will also be added. This will allow tasks to be run in the background while and after
-the response is/has been sent. Please note that `LambdaContext` will not be `Sendable` anymore.
+the response is/has been sent. Please note that `LambdaContext` will not be `Sendable` anymore, because in order
+to allow adding background tasks in a structured way, the `LambdaContext` will need to be backed with a `TaskGroup`.
+Since `TaskGroup` is not `Sendable` we cannot mark `LambdaContext` as `Sendable`. If users need any property out of the
+`LambdaContext` they can just extract the property and then pass it around or close over it as all the properties are
+`Sendable`.
 
 ```swift
 /// A context object passed as part of an invocation in LambdaHandler handle functions.