Improve documentation of X-Forwarded-For header handling
minor #9247 [HttpKernel] Improve documentation of X-Forwarded-For header handling (samuelvogel)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #9247).

Discussion
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[HttpKernel] Improve documentation of 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

Commits
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1f786e0 [HttpKernel] Improve documentation of X-Forwarded-For header handling