man: add an additional note about journalctl -u

Fixes #5387.

I kept the _SYSTEMD_UNIT= example because it is easy to understand and
not very verbose. _SYSTEMD_CGROUP has much longer entries which do not
fit well in the narrow man page. Instead, I added an explanation of what
-u is translated into.

diff --git a/man/journalctl.xml b/man/journalctl.xml
index 39c8c07..be2916c 100644 (file)
--- a/man/journalctl.xml
+++ b/man/journalctl.xml
@@ -930,7 +930,8 @@
     <para>With one match specified, all entries with a field matching
     the expression are shown:</para>
 
-    <programlisting>journalctl _SYSTEMD_UNIT=avahi-daemon.service</programlisting>
+    <programlisting>journalctl _SYSTEMD_UNIT=avahi-daemon.service
+journalctl _SYSTEMD_CGROUP=/user.slice/user-42.slice/session-c1.scope</programlisting>
 
     <para>If two different fields are matched, only entries matching
     both expressions at the same time are shown:</para>
@@ -950,6 +951,19 @@
 
     <programlisting>journalctl _SYSTEMD_UNIT=avahi-daemon.service _PID=28097 + _SYSTEMD_UNIT=dbus.service</programlisting>
 
+    <para>To show all fields emited <emphasis>by</emphasis> a unit and <emphasis>about</emphasis>
+    the unit, option <option>-u</option>/<option>--unit=</option> should be used.
+    <command>journalctl -u <replaceable>name</replaceable></command>
+    expands to a complex filter similar to
+    <programlisting>_SYSTEMD_UNIT=<replaceable>name</replaceable>.service
+  + UNIT=<replaceable>name</replaceable>.service _PID=1
+  + OBJECT_SYSTEMD_UNIT=<replaceable>name</replaceable>.service _UID=0
+  + COREDUMP_UNIT=<replaceable>name</replaceable>.service _UID=0 MESSAGE_ID=fc2e22bc6ee647b6b90729ab34a250b1
+    </programlisting>
+    (see <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+    for an explanation of those patterns).
+    </para>
+
     <para>Show all logs generated by the D-Bus executable:</para>
 
     <programlisting>journalctl /usr/bin/dbus-daemon</programlisting>