List of `completion-styles' overrides for specific categories.
Each override has the shape (CATEGORY . ALIST) where ALIST is
an association list that can specify properties such as:
- `styles': the list of `completion-styles' to use for that category.
- `cycle': the `completion-cycle-threshold' to use for that category.
Categories are symbols such as `buffer' and `file', used when
completing buffer and file names, respectively.