Document Supervisor's detect_blocking_io option (#2761)

Author: mdegat01

docs/api/supervisor/endpoints.md

@@ -3399,6 +3399,7 @@ Returns information about the supervisor
 | diagnostics         | bool or null | Sending diagnostics is enabled                                |
 | addons_repositories | list         | A list of add-on repository URL's as strings                  |
 | auto_update         | bool         | Is auto update enabled for supervisor                         |
+| detect_blocking_io  | bool         | Supervisor raises exceptions for blocking I/O in event loop   |
 
 **Example response:**
 
@@ -3419,7 +3420,8 @@ Returns information about the supervisor
   "debug_block": false,
   "diagnostics": null,
   "addons_repositories": ["https://example.com/addons"],
-  "auto_update": true
+  "auto_update": true,
+  "detect_blocking_io": false
 }
 ```
 
@@ -3476,6 +3478,7 @@ You need to call `/supervisor/reload` after updating the options.
 | logging             | string | Set logging level                                      |
 | addons_repositories | list   | Set a list of URL's as strings for add-on repositories |
 | auto_update         | bool   | Enable/disable auto update for supervisor              |
+| detect_blocking_io  | string | Enable blocking I/O in event loop detection. Valid values are `on`, `off` and `on_at_startup`. |
 
 </ApiEndpoint>