Docs: lego 5, and split DNS with lego's propagation check

docs/acme.md and jarl-lego show lego 5's commands (lego run, which also
renews; lego migrate when upgrading) beside lego 4's. A new section
explains why lego on a host inside a split DNS fails its own propagation
check when the challenge goes into the external view only (JARL_VIEW, a
one-view token, httpreq's /view/...), and the ways round it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

contrib/lego/jarl-lego.sh

@@ -7,12 +7,17 @@
 #
 # Install it, then:
 #   EXEC_PATH=/usr/local/bin/jarl-lego JARL_TOKEN=@/etc/jarl/lego.token \
-#     lego --email you@example.com --dns exec -d example.com -d '*.example.com' run
+#     lego run --accept-tos --email you@example.com --dns exec -d example.com -d '*.example.com'
+#   (lego 4: lego --accept-tos --email ... --dns exec -d ... run, and "renew" to renew)
 #
 # Options (environment):
 #  JARL_SERVER  Jarl server URL, e.g. https://ns1.example.com:8053 (optional if the jarl CLI is configured)
 #  JARL_TOKEN   API token, or @FILE (optional if the jarl CLI is configured)
 #  JARL_VIEW    Only write into this view (optional; default every view that has the zone)
+#               On a host inside a split DNS, lego's own propagation check then can't
+#               see the record: leave it unset, or run lego with
+#               --dns.propagation.disable-ans (lego 5; --dns.propagation-disable-ans in lego 4)
+#               (docs/acme.md, "Split DNS and lego's propagation check").
 #  JARL_BIN     Path of the jarl binary (optional; default "jarl")
 
 JARL_BIN="${JARL_BIN:-jarl}"