init: Modify docs and add release note for 12255

doc/release-notes/release-notes-pr12255.md

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+systemd init file
+=========
+
+The systemd init file (`contrib/init/bitcoind.service`) has been changed to use
+`/var/lib/bitcoind` as the data directory instead of `~bitcoin/.bitcoin`. This
+change makes Bitcoin Core more consistent with other services, and makes the
+systemd init config more consistent with existing Upstart and OpenRC configs.
+
+The configuration, PID, and data directories are now completely managed by
+systemd, which will take care of their creation, permissions, etc. See
+[`systemd.exec (5)`](https://www.freedesktop.org/software/systemd/man/systemd.exec.html#RuntimeDirectory=)
+for more details.
+
+When using the provided init files under `contrib/init`, overriding the
+`datadir` option in `/etc/bitcoin/bitcoin.conf` will have no effect. This is
+because the command line arguments specified in the init files take precedence
+over the options specified in `/etc/bitcoin/bitcoin.conf`.