message: docs/fleetlock: clarify semantics of error values
diff: @@ -95,3 +95,16 @@ Error values carry a type-identifier and a textual description, according to the
  | value | required | human-friendly error description, as a non-empty JSON string |
  This allows clients to show more specific error details to cluster administrators, instead of generic HTTP errors.
  +
  +For example, an error value like the following could be returned on `/v1/pre-reboot` when all available slots are already in use:
  +
  +```json
  +{
  + "kind": "failed_lock_semaphore_full",
  + "value": "semaphore currently full, all slots are locked already"
  +}
  +```
  +
  +Zincati will log this error using the content of `value`, and it will track the `kind` label in metrics.
  +
  +A server MUST ensure that possible values for `kind` have a bounded/small cardinality.
  