[sphinx extension] Source for Description - currently msg for val/trans

[lars-petter-hauge]

For the user of configsuite, I don't think it's obvious that the msg added to the decorators is what will inform the user in the documentation about what is going on. Also, I feel this splits the information about what is going into the documentation quite significantly. All the descriptions are already within the schema dict, the names, required/default/alllownone etc. - and then you would have to go to the function declaration in order to edit documentation regarding transformations/validations..

And those msg looks to be one-liners, while transformations and validations, especially context based ones, could probably need more than that

Let's think about it a bit at least..

Originally posted by @lars-petter-hauge in #80