docs: update docstring

Signed-off-by: F.N. Claessen <felix@seita.nl>

flexmeasures/data/schemas/sensors.py

@@ -188,7 +188,7 @@ def convert(self, value, param, ctx, **kwargs):
 
 
 class QuantityOrSensorValidator(validate.Validator):
-    """Validator which succeeds if the value passed to it is a valid quantity."""
+    """Validator that executes another validator (the one you initialize it with) only on non-Sensor values."""
 
     def __init__(self, original_validator, *, error: str | None = None):
         self.error = error