docs: remove cost, document missing options and improve docs (#653)

source/plugins/activity/metadata.yml

@@ -1,5 +1,4 @@
 name: "📰 Recent activity"
-cost: 1 REST request per 100 events
 category: github
 index: 16
 supports:
@@ -86,4 +85,4 @@ inputs:
     type: array
     format: comma-separated
     default: ""
-    example: my-repo-1, my-repo-2, owner/repo-3 ...
+    example: my-repo-1, my-repo-2, owner/repo-3, ...