Add known bug for content-visibility in Safari (#7255)

Author: knowler

features-json/css-content-visibility.json

@@ -18,7 +18,9 @@
     }
   ],
   "bugs":[
-    
+    {
+      "description": "[WebKit Bug 283846](https://bugs.webkit.org/show_bug.cgi?id=283846): in Safari, the `auto` value does not keep skipped content findable with the find-in-page feature."
+    }
   ],
   "categories":[
     "CSS"