fix: Include hook hints in evaluation options type. (#250)

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## This PR
In section 4.5.1 of hooks it specifies:
```
> `Flag evaluation options` **MAY** contain `hook hints`, a map of data to be provided to hook invocations.
```

But it does not include this in the types.md file, which makes this a
bit confusing.

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Signed-off-by: Ryan Lamb <[email protected]>

specification/types.md

@@ -118,6 +118,7 @@ An enumerated error code represented idiomatically in the implementation languag
 A structure containing the following fields:
 
 - hooks (one or more [hooks](./sections/04-hooks.md), optional)
+- hook hints ([hook hints structure](./sections/04-hooks.md#42-hook-hints), optional)
 
 ### Flag Metadata