Update doc

Author: ota-meshi

docs/rules/no-undef-properties.md

@@ -16,7 +16,8 @@ This rule warns of using undefined properties.
 This rule can help you locate potential errors resulting from misspellings property names, and implicitly added properties.
 
 ::: warning Note
-This rule cannot detect properties defined in other files or components.
+This rule cannot detect properties defined in other files or components.  
+Note that there are many false positives if you are using mixins.
 :::
 
 <eslint-code-block :rules="{'vue/no-undef-properties': ['error']}">