Review retention automation operation labels and descriptions

The retention automation operations (https://github.com/nuxeo/nuxeo/tree/master/modules/platform/nuxeo-retention-core/src/main/java/org/nuxeo/retention/operations) needs to be reviewed:

• Typo on the Retention.AttachRule automation operation label

  "id" : "Retention.AttachRule",  "label" : "Attach Retation Rule",

• We miss a  Retention category to list all the available operations
• The Legal Hold operation is not explicit, and not prefixed by "Retention"

  "id" : "Document.Hold",  "label" : "Hold",