Alerting docs: clarify data source-managed rules for Prometheus (#98378)
* Clarify DS managed rules support only the creation of Mimir and Loki rules
* additional copy changes
* Extend `Manage alerts via Alerting UI` description
* fix capital letter
* further details for `Manage alerts via Alerting UI`
(cherry picked from commit 7771768363)
Co-authored-by: Pepe Cano <825430+ppcano@users.noreply.github.com>