man: don't document systemctl --failed

This effectively reverts 599b632, which
in turn partially reverted 4dc5b82.

The --failed switch is not documented on purpose, since it is redundant
due to --state=failed, which it predates. Due to that it's not
documented in --help either.

We generally try to avoid redundant interfaces, but if we need to keep
them for compatibility we do so, however remove them from documentation
to ensure they are not used in future.

The man page is now changed to include a comment about the fact that
--failed is not documented on purpose. Also, explicitly mention
--state=failed as example for --state.

man/systemctl.xml

@@ -98,9 +98,10 @@ along with systemd; If not, see <http://www.gnu.org/licenses/>.
         <term><option>--state=</option></term>
 
         <listitem>
-        <para>The argument should be a comma-separated list of unit LOAD,
-        SUB, or ACTIVE states. When listing units, show only those
-        in specified states.</para>
+        <para>The argument should be a comma-separated list of unit
+        LOAD, SUB, or ACTIVE states. When listing units, show only
+        those in specified states. Use <option>--state=failed</option>
+        to show only failed units.</para>
         </listitem>
       </varlistentry>
 
@@ -321,14 +322,10 @@ along with systemd; If not, see <http://www.gnu.org/licenses/>.
       <xi:include href="user-system-options.xml" xpointer="user" />
       <xi:include href="user-system-options.xml" xpointer="system" />
 
-      <varlistentry>
-        <term><option>--failed</option></term>
-
-        <listitem>
-          <para>List units in failed state. This is equivalent to
-          <option>--state=failed</option>.</para>
-        </listitem>
-      </varlistentry>
+      <!-- we do not document -failed here, as it has been made
+           redundant by -state=failed, which it predates. To keep
+           things simple we only document the new switch, while
+           keeping the old one around for compatibility only. -->
 
       <varlistentry>
         <term><option>--no-wall</option></term>