docs: Update federation docs for native histograms

Signed-off-by: Ganesh Vernekar <ganeshvern@gmail.com>

docs/federation.md

@@ -8,8 +8,15 @@ sort_rank: 6
 Federation allows a Prometheus server to scrape selected time series from
 another Prometheus server.
 
-_Note about native histograms (experimental feature): Federation does not
-support native histograms yet._
+_Note about native histograms (experimental feature): To scrape native histograms
+via federation, the scraping Prometheus server needs to run with native histograms
+enabled (via the command line flag `--enable-feature=native-histograms`), implying
+that the protobuf format is used for scraping. Should the federated metrics contain
+a mix of different sample types (float64, counter histogram, gauge histogram) for
+the same metric name, the federation payload will contain multiple metric families
+with the same name (but different types). Technically, this violates the rules of
+the protobuf exposition format, but Prometheus is nevertheless able to ingest all
+metrics correctly._
 
 ## Use cases