docs: Superset 6.1 documentation catch-up — batch 2 (#39441)

Co-authored-by: Superset Dev <dev@superset.apache.org>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/admin_docs/configuration/cache.mdx

@@ -138,14 +138,33 @@ THUMBNAIL_CACHE_CONFIG = init_thumbnail_cache
 ```
 
 Using the above example cache keys for dashboards will be `superset_thumb__dashboard__{ID}`. You can
-override the base URL for selenium using:
+override the base URL for Selenium using:
 
 ```
 WEBDRIVER_BASEURL = "https://superset.company.com"
 ```
 
-Additional selenium web drive configuration can be set using `WEBDRIVER_CONFIGURATION`. You can
-implement a custom function to authenticate selenium. The default function uses the `flask-login`
+To control which user account is used for rendering thumbnails and warming up caches, configure
+`THUMBNAIL_EXECUTORS` and `CACHE_WARMUP_EXECUTORS`. Each accepts a list of executor types (which
+resolve to an owner, creator, modifier, or the currently-logged-in user) and/or a `FixedExecutor`
+pinned to a specific username. By default, thumbnails render as the current user
+(`ExecutorType.CURRENT_USER`) and cache warmup runs as the chart/dashboard owner
+(`ExecutorType.OWNER`).
+
+To force both to run as a dedicated service account (`admin` in this example):
+
+```python
+from superset.tasks.types import ExecutorType, FixedExecutor
+
+THUMBNAIL_EXECUTORS = [FixedExecutor("admin")]
+CACHE_WARMUP_EXECUTORS = [FixedExecutor("admin")]
+```
+
+Use a dedicated read-only service account here rather than a personal admin account, so that
+thumbnail rendering and cache warmup tasks don't fail if a specific user's credentials change.
+
+Additional Selenium WebDriver configuration can be set using `WEBDRIVER_CONFIGURATION`. You can
+implement a custom function to authenticate Selenium. The default function uses the `flask-login`
 session cookie. Here's an example of a custom function signature:
 
 ```python