Add secure field to Synthetics Browser Test variables and update docs (#1465)

Co-authored-by: ci.datadog-api-spec <[email protected]>

Author: api-clients-generation-pipeline[bot]

.apigentools-info

@@ -4,13 +4,13 @@
     "spec_versions": {
         "v1": {
             "apigentools_version": "1.6.4",
-            "regenerated": "2023-05-19 14:45:10.283637",
-            "spec_repo_commit": "937a9438"
+            "regenerated": "2023-05-19 19:01:59.374943",
+            "spec_repo_commit": "c3f9f308"
         },
         "v2": {
             "apigentools_version": "1.6.4",
-            "regenerated": "2023-05-19 14:45:10.295797",
-            "spec_repo_commit": "937a9438"
+            "regenerated": "2023-05-19 19:01:59.387350",
+            "spec_repo_commit": "c3f9f308"
         }
     }
 }