Update how to use [WebIDL2JSValueAsUnsupported]

Closes #235.

Author: domenic

README.md

@@ -531,7 +531,7 @@ This is necessary because we need to figure out at code-generation time whether
 
 ### `[WebIDL2JSValueAsUnsupported=value]`
 
-This extended attribute can be applied to named or indexed getters or setters. It says that whether the interface supports a given property name/index can be automatically derived by looking at the return value of its indexed getter/setter: whenever `value` is returned, the name/index is unsupported. Typically, `value` is either `undefined` or `_null`.
+This extended attribute can be applied to named or indexed getters or setters. It says that whether the interface supports a given property name/index can be automatically derived by looking at the return value of its indexed getter/setter: whenever `value` is returned, the name/index is unsupported. Typically, `value` is either `_undefined` or `_null`.
 
 In practice, this means that the implementation class only needs to implement a single method (the named/indexed getter method), and doesn't need to implement the `[idlUtils.supportsPropertyName]()` or `[idlUtils.supportsPropertyIndex]()` method separately.