Documents rest_command header templates usage (#10213)

source/_integrations/rest_command.markdown

@@ -31,7 +31,7 @@ service_name:
     url:
       description: The URL (supports template) for sending request.
       required: true
-      type: [string, template]
+      type: template
     method:
       description: HTTP method to use (get, patch, post, put, or delete).
       required: false
@@ -44,7 +44,7 @@ service_name:
     payload:
       description: A string/template to send with request.
       required: false
-      type: [string, template]
+      type: template
     username:
       description: The username for HTTP authentication.
       required: false
@@ -83,6 +83,7 @@ rest_command:
     headers:
       authorization: !secret rest_headers_secret
       accept: 'application/json, text/html'
+      user-agent: 'Mozilla/5.0 {{ useragent }}'
     payload: '{"profile":{"status_text": "{{ status }}","status_emoji": "{{ emoji }}"}}'
     content_type:  'application/json; charset=utf-8'
     verify_ssl: true