features.md: add a note to avoid confusion about annotations

The "annotations" property in features.md is not expected to
enumerate the possible values of the "annotation" property in config.md.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

features.md

@@ -129,6 +129,8 @@ The `null` value MUST NOT be confused with an empty value such as `0`, `false`,
 **`annotations`** (object, OPTIONAL) contains arbitrary metadata of the runtime.
 This information MAY be structured or unstructured.
 Annotations MUST be a key-value map that follows the same convention as the Key and Values of the [`annotations` property of `config.json`](config.md#annotations).
+However, annotations do not need to contain the possible values of the [`annotations` property of `config.json`](config.md#annotations).
+The current version of the spec do not provide a way to enumerate the possible values of the [`annotations` property of `config.json`](config.md#annotations).
 
 ### Example
 ```json