Revert "Remove all errors when generating reference doc using api-extractor (#1095)" (#1096)

Reverts #1095

It turns out that @hidden -> @internal is not a safe transformation for exported values since our `tsconfig.json` specifies `stripInternal: true`.

Temporarily rolling back change - I'll send out a smaller patch that contains subset of changes in #1095

Author: taeold

docgen/api-extractor.v2.json

@@ -10,25 +10,5 @@
     "enabled": true,
     "reportTempFolder": "<projectFolder>/docgen/v2/temp",
     "reportFolder": "<projectFolder>/docgen/v2"
-  },
-  "messages": {
-    "compilerMessageReporting": {
-      "default": {
-        "logLevel": "warning"
-      }
-    },
-    "extractorMessageReporting": {
-      "default": {
-        "logLevel": "warning"
-      },
-      "ae-missing-release-tag": {
-        "logLevel": "none"
-      }
-    },
-    "tsdocMessageReporting": {
-      "default": {
-        "logLevel": "error"
-      }
-    }
   }
 }