docs: add note about negative multipliers in Prometheus as-collected export (#22321)

* docs: add note about negative multipliers in Prometheus as-collected export Explain that some Netdata dimensions (e.g., cgroup I/O write) use a -1 multiplier by design for AREA charts, causing negative values in the as-collected data source. Recommend average or sum source for positive values. * docs: Added a proper '#### Negative Values in Exported Metrics' subsection under '### Understanding Netdata Metrics' (after '#### Dimensions', before '### Netdata Data Source') explaining the -1 multiplier design, listing all affected chart contexts grounded in cgroup-charts.c and proc_diskstats.c source code, and providing an abs() PromQL example. Removed the prior incorrect :::note inside the As-Collected section that claimed average/sum data sources produce positive values — all data sources reflect the -1 multiplier because it is applied in the RRD layer during collection (confirmed via process_data.c:exporting_calculate_value_from_stored_data reading stored values that already include the multiplier). * docs: Removed the enumerated chart context list per @Ancairon's feedback. The section now explains the -1 multiplier design without listing specific contexts, directing users to identify affected charts from the dashboard view instead. --------- Co-authored-by: nedi-app[bot] <nedi-app[bot]@users.noreply.github.com>