Improve severity docs

Fixes #517 now that I had time to experiment and grasp what was going on.

I got really confused by the fact that the examples were incomplete/invalid, so I tried to improve them and add comments even to make it clear that the severity has to be in the matching groups for the RegexDecoder, and not as I tried it at first setting it in the message_fields. 

Anyway I hope you can agree that this helps.