Namespace-level publish rate limits are poorly documented

**Describe the bug**
Two issues:
1. The documentation for how to set namespace-level publish rate limits is confusing. Its given example is:

```
"publishMaxMessageRate": {
"additionalProp1": {
"publishThrottlingRateInMsg": 0,
"publishThrottlingRateInByte": 0
},
"additionalProp2": {
"publishThrottlingRateInMsg": 0,
"publishThrottlingRateInByte": 0
},
"additionalProp3": {
"publishThrottlingRateInMsg": 0,
"publishThrottlingRateInByte": 0
}
},
```
There is no information as to:
- What `additionalProp1` etc. should be.
- What it means to supply multiple groups of msg/byte rate limits.
- Whether both Msg and Byte must be supplied for each sublimit.

**Expected behavior**
The OpenAPI documentation would contain information indicating how these should be set when namespaces are created.

Start by locating the OpenAPI documentation for namespace creation and inspect the publishMaxMessageRate example. Clarify what each additional property represents, how multiple message/byte limits work, and whether both values are required; done means the documentation answers these questions with a usable example.

Written by the indexing model from the issue text.