amdgpu: Clarify module info for "freesync_video" parameter

The amdgpu driver parameter "freesync_video" I think can benefit from a more clear explanation when using modinfo. Here's a suggestion:

Reports additional video modes with video/film-specific timings, implemented via VRR, even if these modes aren't reported by the display.

Based on the source code, this appears to be what is happening. It just seems the existing explanation using modinfo is ambiguous and not very helpful.