Improve providers documentation.

docs/content/providers/rancher.yml

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+# Enable Rancher Provider.
+providers:
+  rancher:
+
+  # Expose Rancher services by default in Traefik.
+  exposedByDefault: true
+
+  # Enable watch Rancher changes.
+  watch: true
+
+  # Filter services with unhealthy states and inactive states.
+  enableServiceHealthFilter: true
+
+  # Defines the polling interval (in seconds).
+  refreshSeconds: true
+
+  # Poll the Rancher metadata service for changes every `rancher.refreshSeconds`, which is less accurate
+  intervalPoll: false
+
+  # Prefix used for accessing the Rancher metadata service
+  prefix: "/latest"