Key Management API Wiki Page does mention how to authenticate

I had to dig through the e2e tests to figure out that the [Key Management API](https://github.com/Azure/azure-functions-host/wiki/Key-management-API) requires `?code={masterKey}`.

`Authorization: Bearer {DefaultAzureCredentialToken}` nor the header `x-functions-key` worked in my hands.

For example:

```js
const url = `https://myfunctionapp.azurewebsites.net/admin/functions/mySpecialFunction/keys/aKeyNameThatIChoose?code=${hostKeys.masterKey}`;

await fetch(url, { method: 'POST' });
```

Please mention this in the wiki.

Open the Key Management API wiki page linked in the issue and compare its authentication guidance with the e2e tests mentioned in the report. Document that requests require ?code={masterKey}, including the example usage, and make sure the page explains the authentication methods that did not work for the reporter.

Written by the indexing model from the issue text.