Docs: document Battery Maintain SOC and the Time Of Use switch

Clarify that the Time Of Use switch is the inverter master toggle (touAction)
and that Battery Maintain SOC only applies while it is on. List Battery
Maintain SOC in the features.

Author: maxbanton

README.md

@@ -56,11 +56,13 @@ outage, use a local option (RS485 with ESP32, or SolarAssistant) instead.
   from the API, so new fields a firmware exposes show up automatically.
 - Units come straight from the API payload, with correct device and state
   classes, so energy sensors feed the Energy dashboard and long term statistics.
-- Controls exposed as raw inverter parameters (no invented presets): Solar Sell
-  and Time Of Use switches; Max Charge Current, Max Discharge Current, Max Sell
-  Power and Low Battery SOC numbers; and a `set_tou_schedule` service for the
-  Time Of Use table. Names match the inverter and Deye app. Control commands are
-  asynchronous and confirmed against the order status before reporting success.
+- Controls exposed as raw inverter parameters (no invented presets): a Battery
+  Maintain SOC number (the Time Of Use Batt %, your main battery-level control);
+  Low Battery SOC, Max Charge Current, Max Discharge Current and Max Sell Power
+  numbers; Solar Sell and Time Of Use switches; and a `set_tou_schedule` service
+  for the full Time Of Use table. Names match the inverter and Deye app. Control
+  commands are asynchronous and confirmed against the order status before
+  reporting success.
 
 ## Battery charge level from automations
 
@@ -80,8 +82,14 @@ data:
 ```
 
 For full control of individual slots (different % or flags per time window) use
-the `deye_cloud.set_tou_schedule` service, and enable or disable the schedule
-with the **Time Of Use** switch or the `deye_cloud.set_tou_days` service.
+the `deye_cloud.set_tou_schedule` service.
+
+The **Time Of Use** switch is the inverter's Time Of Use master toggle (the
+checkbox on the Work Mode screen, the `touAction` parameter). It must be **on**
+for the schedule, and therefore Battery Maintain SOC, to take effect; turning it
+off makes the inverter ignore the schedule. Use `deye_cloud.set_tou_days` to
+turn it on or off and limit it to specific weekdays. The switch also exposes the
+current 6-slot schedule as its `schedule` attribute.
 
 ## Installation (HACS)