Correct docs for /redact_status failed_redactions/error (#20380)

Spawning from looking at these docs while looking at this area in
https://github.com/element-hq/synapse/pull/20379

changelog.d/20380.doc

@@ -0,0 +1 @@
+Update `/redact_status` admin API docs to reflect better response example and document the `error` field.

docs/admin_api/user_admin_api.md

@@ -1585,8 +1585,8 @@ A response body like the following is returned:
 
 ```
 {
-  "status": "active",
-  "failed_redactions": [],
+  "status": "complete",
+  "failed_redactions": {},
 }
 ```
 
@@ -1601,8 +1601,12 @@ The following parameters should be set in the URL:
 
 The following fields are returned in the JSON response body:
 
-- `status` - string - one of `scheduled`/`active`/`complete`/`failed`, indicating the status of the redaction job
-- `failed_redactions` - dictionary - the keys of the dict are event ids the process was unable to redact, if any, and the values are 
-  the corresponding error that caused the redaction to fail
+- `status` - string (required) - one of `scheduled`/`active`/`complete`/`failed`,
+  indicating the status of the redaction job
+- `failed_redactions` - dictionary (optional) - when the redaction status is `complete`,
+  contains the keys of the dict are event ids the process was unable to redact, if any,
+  and the values are the corresponding error that caused the redaction to fail
+- `error` - string (optional) - when the redaction `status` is `failed`, this field will
+  contains a description of the error encountered during the redaction process.
 
 _Added in Synapse 1.116.0._