Add OTEL Plugin cache response behavior docs

[Guaris]

Definition of done

Add a FAQ or a note to the OTEL plugin that explains when the Proxy Cache cache Advanced plugin returns a cache, the OTEL plugin doesn't emit traces.

Information

https://kongstrong.slack.com/archives/CDSTDSG9J/p1765908236353019

Size

• S (Small). Example: fixing a broken link, updating wording in an FAQ or paragraph