fix(truenas): report usable dataset space

apps/docs/docs/integrations/truenas/index.mdx

@@ -27,6 +27,8 @@ import Alert from '@theme/Admonition';
       ]}
 />
 
+The storage values shown by these widgets use the root ZFS dataset's usable `used` and `available` space, including RAIDZ parity overhead and dataset limits.
+
 ### Adding the integration
 <AddingIntegration />