avformat/hlsenc: better error log message for var_stream_map content

avformat/hlsenc: better error log message for var_stream_map content

When multiple variant streams are specified by var_stream_map option,
%v is expected either in the filename or in the last sub-directory name,
but only in one of them. When both of them contains %v string, current
error message only states half of the truth.
And even %v may appears several times inside the last sub-directory name
or in filename pattern.
This patch clarifies this in the log message and in the doc also.

Signed-off-by: Bela Bodecs <bodecsb@vivanet.hu>