Add field description for Roku host (#104631)

Co-authored-by: Joost Lekkerkerker <joostlek@outlook.com>

homeassistant/components/roku/strings.json

@@ -6,6 +6,9 @@
         "description": "Enter your Roku information.",
         "data": {
           "host": "[%key:common::config_flow::data::host%]"
+        },
+        "data_description": {
+          "host": "The hostname or IP address of the Roku device to control."
         }
       },
       "discovery_confirm": {