docs: Please update the documentation of the ad-hoc filters to clarify the limitations of this feature

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: As of Grafana 11, due to the #84673, the UX of the ad-hoc filters is very disappointing. I started my flow on the Explore page, then chose "Add to the dashboard", then configured ad-hoc filters, then spent an hour figuring out why they didn't work, then found #84673. Since Explore page uses `-- Mixed --` datasource by default, dashboards created from it are incompatible with the ad-hoc filters. Please update the documentation of the ad-hoc filters to clarify the limitations of this feature. **What would you like to be added**: Please document the conditions under which the ad-hoc filters are supported. E.g., > Please be aware that ad-hoc filters are incompatible with the `-- Mixed --` datasource. > > Make sure that a panel doesn't use `-- Mixed --` datasource, because otherwise ad-hoc filters won't be applied to such a panel without giving any warning. **Who is this feature for?** Users who are interested in the ad-hoc filters.