Add example to XML param

[mattfrear]

OpenApi 3.0 supports adding examples to parameters.

https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.1.md#exampleObject

I propose adding support for this via the param XML comments:

/// <param name="keywords" example="hello">A list of search terms</param>
[HttpGet(Name = "SearchProducts")]
public IEnumerable<Product> Get([FromQuery(Name = "kw")]string keywords = "foobar")

This populates the swagger-ui form:

I can submit a PR for this.

Related issue: mattfrear/Swashbuckle.AspNetCore.Filters#66 (comment)