List concurrency in customizable components

Architecture.md

@@ -111,69 +111,72 @@ flowchart TD;
 In particular, many components/features are optional and some can even be fully customized.
 Moreover, the design allows the library to be used alongside other `asyncio` frameworks in the same Python script. We have a dedicated section on this in the [over here](../Frequently-requested-design-patterns#running-ptb-alongside-other-asyncio-frameworks).
 
-## Opt-in and Opt-out components
-
-### Fetching updates
+## Fetching updates
 
 The `Updater` class is there to fetch updates from the Bot API for your bot.
 But you don't need to use it.
 If you want to implement a custom mechanism to fetch updates (e.g. a [[custom webhook setup|Webhooks]], you can just put your updates into the `Application.update_queue` or even manually call `Application.process_update`.
 To build an `Application` that doesn't use an `Updater`, simply pass `None` to `ApplicationBuilder.updater`.
 
-### Scheduling tasks
+## Concurrent update processing
+
+By default, the `Application` handles updates sequentially, i.e. one by one.
+However, you can also process updates concurrently, i.e. multiple updates at the same time.
+It is also possible to customize which updates should be processed concurrently and which should be processed sequentially by providing a custom implementation of the `BaseUpdateProcessor` interface class.
+Please have a look at [[this wiki page|Concurrency]] for more info.
+
+## Scheduling tasks
 
 The `JobQueue` class integrates scheduling logic into the setup of `telegram.ext`.
 This feature is optional.
 If you don't need/want to use a `JobQueue`, simply pass `None` to `ApplicationBuilder.job_queue`.
 
-### Arbitrary `callback_data`
+## Arbitrary `callback_data`
 
 The `CallbackDataCache` is the backend for caching arbitrary `callback_data`.
 This feature must be explicitly activated to be used.
 Please see [[this wiki page|Arbitrary-callback_data]] for more info.
 
-### Default values
+## Default values
 
 The `Defaults` class allows you to specify default values for parameters that appear quite often, e.g. the `parse_mode` parameter.
 This is an opt-in feature.
 Please have a look at [[this wiki page|Adding-defaults-to-your-bot]] for more info.
 
-### Persisting data across restarts
+## Persisting data across restarts
 
 By default, data like `Application.{chat, bot, user}_data` is stored in-memory and is lost when the bot shuts down.
 PTB includes an optional functionality to persist data across restarts, which has `BasePersistence` as interface class at it's core.
 Head to [[this page|Making-your-bot-persistent]] for more info.
 
 
-### Rate limiting
+## Rate limiting
 
 By default, all requests to the bot API, i.e. calling `Bot.send_message`, `Message.reply_text` or any other API method, are immediately forwarded to Telegram.
 Since Telegram imposes rate limits, this can lead to flood errors if you're making to many requests in a short time span.
 PTB includes an optional functionality to "throttle" the outgoing requests, which has `BaseRateLimiter` as interface class at it's core.
 Head to [[this page|Avoiding-flood-limits]] for more info.
 
-## Customizing Components
-
-### Networking
+## Networking
 
 By default, PTB will use the `httpx` library for the networking backend, i.e. making requests to the Bot API.
 However, you are free to use a custom backend implementation as well.
 For this, you'll have to implement the `BaseRequest` interface class and pass two instances of your custom networking class to
 `ApplicationBuilder.request` and `ApplicationBuilder.get_updates_request`.
 
-### Update handlers
+## Update handlers
 
 The `Handler` interface class is the most import class when it comes to the question of how the `Application` processes updates.
 PTB comes with a number of [[built-in handler|Types-of-Handlers]] which cover most important use cases.
 However, if you want to implement a custom logic of when an update should be handled, you can also write a custom implementation of `Handler` and use that.
 
-### Persistence
+## Persistence
 
 As mentioned above, PTBs persistence functionality is based on the interface class `BasePersistence`.
 To use persistence in PTB, you use an implementation of this class.
 PTB comes already ships two implementations (see [[this page|Making-your-bot-persistent]]), but you are very welcome to implement a persistence class for your own serialization backend.
 
-### Customizing the `context` parameter
+## Customizing the `context` parameter
 
 The class `CallbackContext` is a central part of the handler/job callbacks in PTB and more advanced users may want to add custom behavior to this class.
 You can do so by passing an instance of the `ContextTypes` to `ApplicationBuilder.context_types`.