docs: Documented Bunyan log levels (#3444)

lib/config/definitions.js

@@ -92,6 +92,7 @@ const options = [
     description: 'Logging level',
     stage: 'global',
     type: 'string',
+    allowedValues: ['fatal', 'error', 'warn', 'info', 'debug', 'trace'],
     default: 'info',
     env: 'LOG_LEVEL',
   },

renovate-schema.json

@@ -56,6 +56,7 @@
     "logLevel": {
       "description": "Logging level",
       "type": "string",
+      "enum": ["fatal", "error", "warn", "info", "debug", "trace"],
       "default": "info"
     },
     "logFile": {