Update docs for new FeatureAttribute behaviour (#1365)

Co-authored-by: Teemu R. <tpr@iki.fi>

docs/source/featureattributes.md

@@ -0,0 +1,13 @@
+Some modules have attributes that may not be supported by the device.
+These attributes will be annotated with a `FeatureAttribute` return type.
+For example:
+
+```py
+    @property
+    def hsv(self) -> Annotated[HSV, FeatureAttribute()]:
+        """Return the current HSV state of the bulb."""
+```
+
+You can test whether a `FeatureAttribute` is supported by the device with {meth}`kasa.Module.has_feature`
+or {meth}`kasa.Module.get_feature` which will return `None` if not supported.
+Calling these methods on attributes not annotated with a `FeatureAttribute` return type will return an error.