man: improve wording of systemctl's --after/--before

Commit 4a77ca7 was an attempt at fixing the wording of --after and --before,
but the new wording was unclear.

Split the combined --after/--before section into a separate section for
each, explicitly state what each option does, and add information about
how these lists are generated.

Reported-by: Andrey Borzenkov <arvidjaar@gmail.com>
Reported-by: Lennart Poettering <lennart@poettering.net>
(cherry picked from commit 9029f64298)
(cherry picked from commit 7b6a54f753)

man/systemctl.xml

@@ -173,12 +173,27 @@ systemctl start foo
 
       <varlistentry>
         <term><option>--after</option></term>
+
+        <listitem>
+          <para>With <command>list-dependencies</command>, show the
+          units that are ordered before the specified unit. In other
+          words, list the units that are in the <varname>After=</varname>
+          directive of the specified unit, have the specified unit in
+          their <varname>Before=</varname> directive, or are otherwise
+          implicit dependencies of the specified unit.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
         <term><option>--before</option></term>
 
         <listitem>
-          <para>Show after (before) which units the specified unit is started
-          with <command>list-dependencies</command>.
-          </para>
+          <para>With <command>list-dependencies</command>, show the
+          units that are ordered after the specified unit. In other
+          words, list the units that are in the <varname>Before=</varname>
+          directive of the specified unit, have the specified unit in
+          their <varname>After=</varname> directive, or otherwise depend
+          on the specified unit.</para>
         </listitem>
       </varlistentry>