docs: add info about remotewrite client config in ruler

[nickandreev]

What this PR does / why we need it:
When setting up a remote write client for recording rules, I found it unclear what RemoteWriteConfig referred to. After checking the code, I realised it was actually a Prometheus remote write configuration. Adding this clarification in the documentation will make it easier for others to understand the setup without needing to dive into the code.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.

[CLAassistant]

All committers have signed the CLA.