Allow enum on parameter declarations

[erunion]

* @api-param:private grant_type (enum, required) - Grant type. Must be set to "google".
*      + Members
*          - `google`

With this parameter declaration, API Blueprint for this will be generated as:

 - `grant_type` (enum[enum[string]], required) - Grant type. Must be set to "google".
    + Members
        + `google`

Since having Members present, Mill automatically infers the parameter as an enum[string], we should just allow enum for doing the same.