Add small description of each category in notifications

[yrodiere]

Currently notifications are a bit raw and newcomers might get lost.

It could be nice to add a description of each category in the notification, along with (maybe) some tips about what to do with those issues. Something like what we include in the documentation, but as short as possible.

The description shouldn't be obtrusive, so it would be a good idea to make its text smaller than the bullet points.