Add [smart radiator thermostat] description for Switchbot Cloud (#41053)

Co-authored-by: c0ffeeca7 <38767475+c0ffeeca7@users.noreply.github.com>

source/_integrations/switchbot_cloud.markdown

@@ -129,6 +129,10 @@ Please note, device names configured in the SwitchBot app are transferred into H
 - [Air Purifier](https://www.switch-bot.com/products/switchbot-air-purifier)
 - [Air Purifier Table](https://www.switch-bot.com/products/switchbot-air-purifier-table)
 
+### Water heater
+
+- [Smart Radiator Thermostat](https://www.switch-bot.com/products/switchbot-smart-radiator-thermostat)
+
 ## Supported functionality
 
 ### Plugs and switches
@@ -429,6 +433,19 @@ Features:
 - return to base
 - get battery
 
+
+### Water Heater
+
+#### Smart Radiator Thermostat
+
+Features:
+- turn on
+- turn off
+- set temperature
+- get temperature
+- set mode
+- get battery
+
 ## Important considerations
 
 {% note %}