servo: bitmagnet: add docs

hosts/by-name/servo/services/bitmagnet.nix

@@ -1,3 +1,14 @@
+# bitmagnet is a DHT crawler. it discovers publicly reachable torrents and indexes:
+# - torrent's magnet URI
+# - torrent's name
+# - torrent's file list (the first 100 files, per torrent), including size and "type" (e.g. video)
+# - seeder/leecher counts
+# - torrent's size
+# it provides a web UI to query these, especially a search form.
+# data is stored in postgresql as `bitmagnet` db (`sudo -u bitmagnet psql`)
+# after 30 days of operation:
+# - 12m torrents discovered
+# - 77GB database size  => 6500B per torrent
 { config, ... }:
 {
   services.bitmagnet.enable = true;