Gateway doc needs improvements regarding inter Gateway listener conflicts

**What would you like to be added**:
Clarify on our docs and API docs what should happen when 2 distinct Gateways have a conflict on a listener. Per the discussion on our community meeting: https://youtu.be/AShtmkJXAR8?si=qFA86Z21O7ncoqsL&t=1032

- Two Gateways CAN have the same listeners, even if they conflict, as each gateway should be an independent instance
- Implementations that merge all of their listeners on a single proxy instance SHOULD work internally on the listener isolation
- Implementations that merge all of their listeners on a single proxy but can't isolate the bind/listener MUST add a condition to the Gateway (this is Implementation Specific)

**Why this is needed**:

Clarify when conflicts may exist

/cc @candita @youngnick

Start by reviewing the project documentation and API documentation areas, then read the linked community-meeting discussion for the intended listener-conflict behavior. Update both documentation surfaces to clarify the three stated cases, including when an implementation-specific Gateway condition is required; the work is done when conflicts between distinct Gateways and listener isolation are unambiguous.

Written by the indexing model from the issue text.