GAMMA: backendRef port and parentRef clarification

When searching through the docs, it never specifies if a backendRef has port, it must use the same port as the parentRef service. From the docs, it seems different ports are allowed, but not explicitly said.
There is also no conformance tests testing if the ports are different 👀

Locate the documentation describing backendRef and parentRef, then review existing conformance tests for port handling. Clarify whether differing ports are allowed in the relevant docs and add conformance coverage for that case; done means the documentation and test behavior agree.

Written by the indexing model from the issue text.