[HttpKernel] Improve documentation X-Forwarded-For header handling

After having problems with the handling of `X-Forwarded-For` headers and the configuration of trusted proxies, it was really hard to understand how the algorithm actually works. After looking into it, the PHPDoc of `IpUtils` did not really describe what the methods do. For `Request::getClientIps()` the PHPDoc actually explained how the `X-Forwarded-For` header works with multiple proxies, but it's really hard to understand, when it's not mentioned what the text is actually talking about. As one would expect a description of what the method does, I altered the description to describe the return value.

Feel free not to merge the inline comments in `Request::getClientIps()`, even though they greatly help understanding this method for somebody not deeply familiar with the internals of `HttpKernel` and HTTP proxy handling in general.

| Q             | A
| ------------- | ---
| Fixed tickets | none
| License       | MIT