lavfi/psnr: rename 's' parameter to match documentation

Also removes some irrelevant lines in documentation.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
ff_lpc_calc_coefs: assert that the type is levinson or cholesky

Otherwise the code could misbehave (and there are no other types anyway)
Fixes CID700759

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>