milestone 24: persist and surface the unsupported-line count

docs/reference/configuration.md

@@ -287,6 +287,21 @@ first, so a downloaded list can reopen only a hole another downloaded list dug.
 Each source reports how many it carried as `exceptions`; there is no way to write
 one by hand, and no reason to want one — write an allow rule instead.
 
+Two counters report what a compile skipped, and they are different facts.
+`skipped_regex` counts regex lines: nxdns has a regex engine, but it takes
+patterns only from the operator, so a regex line in a downloaded list is counted,
+skipped and surfaced — adopt the ones you trust as `regex` rules.
+`skipped_unsupported` counts lines nxdns cannot safely translate into a DNS
+decision: cosmetic element hiding (`##`, `#@#`, `#?#`), rules carrying a `$`
+modifier (except `$important` on an exception line, tolerated above), scheme
+anchors, non-anchored `@@` forms — and, in a `domains`-format
+list, a line holding more than one field before its inline comment, which usually
+means the list is really a hosts file that was declared as `domains`. Neither is
+an error, and the two are never one number. A large `skipped_unsupported` beside
+a small `domain_count` usually means the list is written for browser extensions,
+and its DNS or hosts variant will block more here. Both appear per source in the
+blocklists UI and on `/api/blocklists`.
+
 ### group_sources
 
 Which groups consult which blocklist sources.