ec2_asg: adding explanation of replace_all_instances (#46694)

* adding explanation of replace_all_instances

How does "replace_all_instances" work? does it increase the number of instances by 1 in the Load balancer and it scales in a server and waits for it to complete its health check, OR   does it terminate a node a that causes an autoscale event to happen and then adds a new server?
+label: docsite_pr

lib/ansible/modules/cloud/amazon/ec2_asg.py

@@ -82,7 +82,10 @@ options:
       - Desired number of instances in group, if unspecified then the current group value will be used.
   replace_all_instances:
     description:
-      - In a rolling fashion, replace all instances with an old launch configuration with one from the current launch configuration.
+      - In a rolling fashion, replace all instances that used the old launch configuration with one from the new launch configuration.
+        It increases the ASG size by C(replace_batch_size), waits for the new instances to be up and running.
+        After that, it terminates a batch of old instances, waits for the replacements, and repeats, until all old instances are replaced.
+        Once that's done the ASG size is reduced back to the expected size.
     version_added: "1.8"
     default: 'no'
   replace_batch_size: