Previously, "dar" and "a" were documented like "input display aspect
ratio", but their actual value was "in_w/in_h".

In order to avoid to break scripts which rely on the "a" variable, the
patch keeps the same semantics but fixes the corresponding docs, and
fixes the semantics of the recently added "dar" variable, which
now correctly expresses the input Display Aspect Ratio value as
"(inw_w/in_h)*sar".