add a note about Symbol compat

Author: yyx990803

src/v2/api/index.md

@@ -906,7 +906,7 @@ All lifecycle hooks automatically have their `this` context bound to the instanc
 
   This pair of options are used together to allow an ancestor component to serve as a dependency injector for its all descendants, regardless of how deep the component hierarchy is. If you are familiar with React, this is very similar to React's context feature.
 
-  The `provide` option should be an object or a function that returns an object. This object contains the properties that are available for injection into its descendants. You can use ES2015 Symbols as keys in this object.
+  The `provide` option should be an object or a function that returns an object. This object contains the properties that are available for injection into its descendants. You can use ES2015 Symbols as keys in this object, but only in environments that natively support `Symbol` and `Reflect.ownKeys`.
 
   The `inject` options should be either an Array of strings or an object where the keys stand for the local binding name, and the value being the key (string or Symbol) to search for in available injections.