man: Add documentation for functionfs socket activation

man/systemd.service.xml

@@ -903,6 +903,22 @@
         and no job queued or being executed for it.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>USBFunctionDescriptors=</varname></term>
+        <listitem><para>Configure the location of file containing
+        FunctionFS descriptors. This is is used only when socket with
+        <varname>ListenUSBFunction</varname> line want to activate this service. Content of
+        this file is writen to ep0 file after it is opened. This is required
+        for socket activation using <varname>ListenUSBFunction</varname>
+        (i.e. for passing all ffs endpoints to service).</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>USBFunctionStrings=</varname></term>
+        <listitem><para>Configure the location of file containing FunctionFS strings.
+        Behavior is similar to <varname>USBFunctionDescriptors</varname>.</para></listitem>
+      </varlistentry>
+
     </variablelist>
 
     <para>Check

man/systemd.socket.xml

@@ -260,6 +260,17 @@
         can be inherited between processes.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>ListenUSBFunction=</varname></term>
+        <listitem><para>Specifies a functionfs endpoint location
+        to listen on. This expects an absolute file system path as
+        argument. Behavior otherwise is very similar to the
+        <varname>ListenFIFO=</varname> directive above. Use this to
+        open functionfs endpoint ep0. When using this option, activated
+        service has to have <varname>USBFunctionDescriptors</varname>
+        and <varname>USBFunctionStrings</varname> options set.</para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>BindIPv6Only=</varname></term>
         <listitem><para>Takes a one of <option>default</option>,