docs: apm-server compatibility with L7 load balancers

[carsonip]

As apm-server handles both http and grpc traffic at the same port using https://github.com/elastic/gmux , it may present a compatibility issue with L7 load balancers which handle http and grpc traffic separately, e.g. nginx k8s ingress. This will mostly affect otlp grpc endpoints. Document this potential issue.