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>

doc/ChangeLog.md

@@ -8,6 +8,12 @@ All relevant changes are documented in this file.
 
 ### Changes
 
+- New `provides` setting for run/task/service/sysv blocks, naming
+  conditions the service asserts in addition to its own `pid/<ident>`.
+  Lets variants of one service, qualified with `if`, share the barrier
+  condition downstream services wait for, which a block title alone
+  could not express.  Any namespace is allowed, and a second claim on
+  the same condition is refused with a warning naming the owner
 - The `command` setting in a block takes a list of candidates for the
   same service, `command = { "/lib/systemd/systemd-udevd", "-udevd" }`,
   and Finit starts the first one it finds.  The line-based format could