man: import org.freedesktop.systemd1(3) from the wiki

As usual, the formatting was fixed and various obvious updates
were done, but nothing major.

I removed documentation of snapshots and related methods though.

TODO

@@ -4,6 +4,9 @@ Bugfixes:
   manager or system manager can be always set. It would be better to reject
   them when parsing config.
 
+* busctl prints errors to stdout:
+  busctl tree org.freedesktop.systemd1  /org/freedesktop/systemd1
+
 External:
 
 * Fedora: add an rpmlint check that verifies that all unit files in the RPM are listed in %systemd_post macros.

man/rules/meson.build

@@ -50,6 +50,7 @@ manpages = [
  ['org.freedesktop.login1', '5', [], 'ENABLE_LOGIND'],
  ['org.freedesktop.machine1', '5', [], 'ENABLE_MACHINED'],
  ['org.freedesktop.resolve1', '5', [], 'ENABLE_RESOLVE'],
+ ['org.freedesktop.systemd1', '5', [], ''],
  ['org.freedesktop.timedate1', '5', [], 'ENABLE_TIMEDATED'],
  ['os-release', '5', [], ''],
  ['pam_systemd', '8', [], 'HAVE_PAM'],

man/systemd.xml

@@ -280,6 +280,9 @@
     files or parameters passed on the kernel command line. For details, see
     <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
 
+    <para>The D-Bus API of <command>systemd</command> is described in
+    <citerefentry><refentrytitle>org.freedesktop.systemd1</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+
     <para>Systems which invoke systemd in a container or initrd environment should implement the <ulink
     url="https://systemd.io/CONTAINER_INTERFACE">Container Interface</ulink> or <ulink
     url="https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface">initrd Interface</ulink>
@@ -1252,6 +1255,7 @@
       <citerefentry><refentrytitle>systemd-notify</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>daemon</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>org.freedesktop.systemd1</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
       <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,