m doc + do not override fadein_time or fadeout_time with None if fadein_steps or fadeout_steps, respectively, is non-positive

Signed-off-by: Mattias Andrée <maandree@operamail.com>
document

Signed-off-by: Mattias Andrée <maandree@operamail.com>
document

Signed-off-by: Mattias Andrée <maandree@operamail.com>