Shape fields doc comment reflection

Please allow reflection on shape field doc comment in order to be able to create
auto documented rest services.

example:

```
const type abc = shape(
/**
* field purpose
*/
'field1' => string
);
```

maybe something like this.

```
$ref = new \ReflectionTypeAlias('abc')->getField('field1')->getDocComment();
```

The issue centers on Hack shape fields and the proposed ReflectionTypeAlias('abc')->getField('field1')->getDocComment() entry point. Start by locating the shape-field reflection and type-alias implementation; done means the doc comment for field1 can be retrieved for generating API documentation.

Written by the indexing model from the issue text.