Update 4xx-client-error.mdx

[cf-aponting]

re: 408 errors, nginx-ssl will return a 408 error if either a client request header takes >15s to be received at our edge, or if the client body takes >60s - so our documentation is incorrect to state that Cloudflare never returns 408s/they're always proxied from the origin, and this has resulted in customer confusion in at least one recent case.

refer to eg: https://wiki.cfdata.org/display/~gcorral/HTTP+408+Error for further specifics

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Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[hyperlint-ai]

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PR Change Summary

Updated documentation to clarify the conditions under which Cloudflare may return a 408 error, correcting previous inaccuracies.

• Clarified that Cloudflare may return a 408 error under specific timeout conditions.
• Corrected the statement regarding 408 errors being solely proxied from the origin.
• Emphasized the importance of reviewing origin server timeout settings.

Modified Files

• src/content/docs/support/troubleshooting/http-status-codes/4xx-client-error.mdx

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