Update letsencrypt.py

Extend `remaining_days` description.

lib/ansible/modules/extras/web_infrastructure/letsencrypt.py

@@ -103,8 +103,10 @@ options:
     alias: ['cert']
   remaining_days:
     description:
-      - "The number of days the certificate must have left being valid before it
-         will be renewed."
+      - "The number of days the certificate must have left being valid.
+         If C(remaining_days < cert_days), then it will be renewed.
+         If the certificate is not renewed, module return values will not
+         include C(challenge_data)."         
     required: false
     default: 10
 '''