[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>
1 file changed