doc: document the provides setting

The migration guide told anyone holding the repeated-stanza idiom for a
per-platform service to split the variants across files or stay on the
line-based format, because a block title is an identity and the
variants have to share one barrier.  provides is the answer, so the
guide converts that shape now instead of routing around it, and the
header of 10-hotplug.conf.in no longer points at the workaround.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

system/10-hotplug.conf.in

@@ -21,9 +21,21 @@
 # The mdev and mdevd systems wait for a different coldplug condition, so they
 # need their own block, and a block title is a service identity: two blocks
 # titled syslogd in one file are two declarations of one service, which Finit
-# rejects.  Give each variant its own file, or keep them as one-liners in the
-# line-based format, which has no titles and allows the repetition.  See
-# doc/config/migration.md for both.
+# rejects.  Give each variant its own title and name the shared barrier that
+# everything downstream waits for:
+#
+#    service syslogd:udev {
+#        if         = "udevd"
+#        conditions = { "run/udevadm:5/success" }
+#        provides   = "pid/syslogd"
+#        command    = "syslogd -F"
+#    }
+#    service syslogd:mdev {
+#        if         = "mdev"
+#        conditions = { "run/coldplug/success" }
+#        provides   = "pid/syslogd"
+#        command    = "syslogd -F"
+#    }
 #
 # Override this file by copying it to /etc/finit.d/, using the same name, then
 # change the contents any way you like, it can even be empty.