Update documentation on the MediaType used for ProblemDetail

[rstoyanchev]

As stated in #29588 (comment), the documentation says:

- For content negotiation, the Jackson `HttpMessageConverter` prefers
"application/problem+json" over "application/json" when rendering a `ProblemDetail`,
and also falls back on it if no compatible media type is found.

However, the actual behavior in our Jackson message support is to always use "application/problem+json".