Improve Documentation Around X-Forwarded- Headers

[rwinch]

Affects: 6.0.12

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Spring could improve the documentation (both reference and Javadoc) around X-Forwarded-* headers. In particular, the X-Forwarded-Prefix header could use some clarifications as demonstrated in #31241

I've discussed this with @rstoyanchev and I'm happy to send a PR with my comment as a starting point for the updated documentation.

[snicoll]

Closing in favor of PR #31491