Update XML documentation - REQUEST_XML & RESPONSE_XML

[sts]

Unlike ModSecurity, REQUEST_XML is an alias for XML, as we also support RESPONSE_XML. That’s because of response body processing. Coraza only supports two XPATH expressions, //@* and /* which are hardcoded. That’s because of the lack of libxml2 support.

https://coraza.io/docs/seclang/variables/#xml

[jptosso]

Also it's not supported by the tinygo version (proxy wasm)

[jcchavezs]

@sts are you up work on the update?