minor #12535 Middleware clarification about being called on "receive" (weaverryan)

This PR was squashed before being merged into the 4.3 branch (closes #12535).

Discussion
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Middleware clarification about being called on "receive"

Hi!

A clarification that middleware may be called twice. I think this is the best spot, and isn't too big or too small.

Commits
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b9f315f Middleware clarification about being called on \"receive\"

Author: javiereguiluz

components/messenger.rst

@@ -54,7 +54,9 @@ Concepts
    For instance: logging, validating a message, starting a transaction, ...
    They are also responsible for calling the next middleware in the chain,
    which means they can tweak the envelope, by adding stamps to it or even
-   replacing it, as well as interrupt the middleware chain.
+   replacing it, as well as interrupt the middleware chain. Middleware are called
+   both when a message is originally dispatched and again later when a message
+   is received from a tansport,
 
 **Envelope**
    Messenger specific concept, it gives full flexibility inside the message bus,
@@ -174,6 +176,8 @@ Hence you can inspect the envelope content and its stamps, or add any::
 
                 // You could for example add another stamp.
                 $envelope = $envelope->with(new AnotherStamp(/* ... */));
+            } else {
+                // Message was just originally dispatched
             }
 
             return $stack->next()->handle($envelope, $stack);

messenger.rst

@@ -1297,6 +1297,10 @@ for each bus looks like this:
     These middleware names are actually shortcuts names. The real service ids
     are prefixed with ``messenger.middleware.``.
 
+The middleware are executed when the message is dispatched but *also* again when
+a message is received via the worker (for messages that were sent to a transport
+to be handled asynchronously). Keep this in mind if you create your own middleware.
+
 You can add your own middleware to this list, or completely disable the default
 middleware and *only* include your own: