[Manual] Document soft lock for FM

After the recent soft lock implementation for FM, this adds the
related information in the manual for Fuze+ and Clip/clip+ (basicaly it's a
 copy/paste from WPS section).
Change-Id: I23c39d7b3763eb2b9897187baa3a9b2663003cf7
Reviewed-on: http://gerrit.rockbox.org/495
Reviewed-by: Amaury Pouly <amaury.pouly@gmail.com>

manual/main_menu/fmradio.tex

@@ -89,6 +89,14 @@
           \ActionFMMenu
           \opt{HAVEREMOTEKEYMAP}{& \ActionRCFMMenu}
           & Display the FM radio settings menu.\\
+          %
+          % software hold targets
+          \nopt{hold_button}{%
+            \opt{SANSA_CLIP_PAD}{\ButtonHome+\ButtonSelect}
+            \opt{SANSA_FUZEPLUS_PAD}{\ButtonPower}
+            \opt{ONDIO_PAD}{\ButtonMenu+\ButtonDown}
+            & Key lock (software hold switch) on/off.\\
+          }%
        \end{btnmap}
 
   \begin{description}