Document changes in inject definition for feature vuejs/vue#6097 (#1069)

* close #6097: Allow defining optional inject dependency with default values

Document changes in inject definition for feature #6097

* reformatted description of inject options

* further clarification of inject options description

Author: pdanpdan

src/v2/api/index.md

@@ -1008,7 +1008,13 @@ type: api
 
   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.
+  The `inject` option should be either:
+  - an array of strings, or
+  - an object where the keys are the local binding name and the value is either:
+    - the key (string or Symbol) to search for in available injections, or
+    - an object where:
+      - the `name` property is the key (string or Symbol) to search for in available injections, and
+      - the `default` property is used as fallback value
 
   > Note: the `provide` and `inject` bindings are NOT reactive. This is intentional. However, if you pass down an observed object, properties on that object do remain reactive.