Incorrect logic description in the HTTPHeaderName section

**What would you like to be added**:

[Documentation for the HTTPHeaderName](https://gateway-api.sigs.k8s.io/reference/1.4/spec/#httpheader) states that

> If multiple entries specify equivalent header names, the first entry with an equivalent name MUST be considered for a match.

Actual behavior is that filters specifying equivalent header names are invalid, for example:

```
requestHeaderModifier:
add:
- name: my-header-name
value: my-header-value
- name: my-header-name
value: my-header-value2
```

So, if multiple entries specify equivalent header names, the respective HTTP/GRPCRoute will fail with `Duplicate value: map[string]interface {}{"name":"my-header-name"}`, so I assume the documentation should be aligned [with this description](https://github.com/kubernetes-sigs/gateway-api/pull/3778/changes)

Start with the HTTPHeaderName section in the linked Gateway API reference and compare its statement with the behavior described in this issue. Read the changes in PR 3778 for the intended wording, then update the documentation so it reflects that equivalent header names are rejected; done means the section matches the documented validation behavior.

Written by the indexing model from the issue text.