add missing param description in various docs (#14849)

source/_integrations/mailgun.markdown

@@ -74,19 +74,11 @@ notify:
 ```
 
 {% configuration %}
-domain:
-  description: This is the domain name to be used when sending out mail.
-  required: true
-  type: string
-sandbox:
-  description: "(**Deprecated**) If a sandboxed domain is used, specify it in `domain`."
+name:
+  description: "The optional parameter name allows multiple notifiers to be created. The notifier will bind to the service notify.NOTIFIER_NAME."
   required: false
-  default: false
-  type: boolean
-api_key:
-  description: This is the API Key that has been generated in your Mailgun account.
-  required: true
   type: string
+  default: notify
 recipient:
   description: The email address of the recipient.
   required: true

source/_integrations/mastodon.markdown

@@ -30,6 +30,11 @@ notify:
 ```
 
 {% configuration %}
+name:
+  description: "The optional parameter name allows multiple notifiers to be created. The notifier will bind to the service notify.NOTIFIER_NAME."
+  required: false
+  type: string
+  default: notify
 access_token:
   description: Your Mastodon access token.
   required: true

source/_integrations/yessssms.markdown

@@ -50,6 +50,11 @@ notify:
 ```
 
 {% configuration %}
+name:
+  description: "The optional parameter name allows multiple notifiers to be created. The notifier will bind to the service notify.NOTIFIER_NAME."
+  required: false
+  type: string
+  default: notify
 username:
   description: This is your login name (usually your phone number). Veryfy that you can use your credentials on the Yesss.at website.
   required: true