[BUG] Microsoft.Billing provider missing documentation for the vast majority of it's endpoints

### API Spec link

N/A, it's missing.

### API Spec version

All

### Describe the bug

Large portions of the `Microsoft.Billing` provider appear to be missing.
For example, I would expect the spec to match with all of the endpoints returned by [the operations endpoint](https://management.azure.com/providers/Microsoft.Billing/operations?api-version=2022-10-01-privatepreview).

This functionality is required to programmatically interact with Billing accounts and their various child objects such as Departments, Enrollment Accounts and similar.

### Expected behavior

For the `Microsoft.Billing` providers to be populated with all valid REST endpoints.

That includes but is not limited to:

* `Microsoft.Billing/billingAccounts/${billingAccountId}/addDepartment`
* `Microsoft.Billing/billingAccounts/${billingAccountId}/departments/write`
* `Microsoft.Billing/billingAccounts/${billingAccountId}/addEnrollmentAccount/write`

And many other similar endpoints that exist and are actively used by the Azure Portal to provide this functionality.

### Actual behavior

`Microsoft.Billing` is lacking nearly all endpoints.

### Reproduction Steps

N/A

### Environment

_No response_