advanced: add a link to the instruction for using AMD AMF encoders in VLC

This merge request adds the link to the documentation how to use AMD AMF encoders in VLC. AMF encoders support was merged into VLC repo recently videolan/vlc!2831 (merged), so we decided to provide instruction how to use these AMF encoders on AMD graphics cards: https://github.com/GPUOpen-LibrariesAndSDKs/AMF/wiki/VLC%20and%20AMF

I'm not sure whether the page "Unusual cases" is suitable place for the link to the AMF documentation, so you might propose better place for this information.