L2E: Update the description of allow_alt_ref
L2E: Update the description of allow_alt_ref

It is fixed per each encoding and can not be changed per GOP.

Change-Id: I5905b712437142f2274bfa674ceef6093495457f

webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx