Mention algorithms from w3c#429 in "Extensibility Definition Requirem…

…ents".

The new algorithms were defined in the "Sensor Type" section, but it makes
sense to list them when describing what extension specifications may do.

index.bs

@@ -2066,6 +2066,11 @@ for each [=sensor types=]:
     For [=sensor types=] where multiple [=device sensor|sensors=] are common,
     [=extension specifications=] may choose not to define a [=default sensor=],
     especially when doing so would not make sense.
+-   A [=threshold check algorithm=], which returns a boolean indicating whether
+    two sensor readings differ enough to cause the [=latest readings=] map to
+    be updated.
+-   A [=reading quantization algorithm=], which may return a sensor reading
+    with reduced accuracy.
 
 <h3 id="extend-automation">Automation</h3>