Added more explanation to keypair. (#52766)

* Added more explanation to keypair. 

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+label: docsite_pr

* Update ec2.py

* Update ec2.py

lib/ansible/modules/cloud/amazon/ec2.py

@@ -29,7 +29,8 @@ version_added: "0.9"
 options:
   key_name:
     description:
-      - Key pair to use on the instance.
+      - key pair to use on the instance. The SSH key must exist on AWS in order to use this argument. If you want to generate keys from Ansible,
+        take a look at `ec2_key` module.
     aliases: ['keypair']
     type: str
   id: