chore: add subrequest note to NIC policies when WAF is enabled

[AlexFenlon]

Add notices across NGINX Ingress Controller policy and WAF documentation to clarify that subrequests (e.g. externalAuth, remote JWKS, apiKey, OIDC, cache background update) do not work when WAF is enabled.

Proposed changes

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[JTorreG]

approved with nits

[pdabelf5]

The language seems a bit strong. WAF & Auth policies do work together, you might get inconsistent results though. It might be better to suggest users avoid using combinations of WAF & Auth policies

removing my approval as the text needs to be revised