Enhance battery pool documentation (#425)

Author: matthias-wende-frequenz

src/frequenz/sdk/timeseries/battery_pool/battery_pool.py

@@ -374,6 +374,9 @@ def capacity(self) -> MetricAggregator[CapacityMetrics]:
     def power_bounds(self) -> MetricAggregator[PowerMetrics]:
         """Get receiver to receive new power bounds when they change.
 
+        Power bounds refer to the min and max power that a battery can
+        discharge or charge at and is also denoted as SoP.
+
         Power bounds formulas are described in the receiver return type.
         None will be send if there is no component to calculate metrics.