Explain how to enable debug for multiple modules

Author: t-bltg

stdlib/Logging/docs/src/index.md

@@ -217,6 +217,9 @@ julia> foo()
 
 ```
 
+Use a comma separator to enable debug for multiple modules: 
+`JULIA_DEBUG=loading,Main`.
+
 ## Examples
 
 ### Example: Writing log events to a file