It currently wrongly documents that it is only called before reading
streams (that has never been the case and all MediaPeriods already need
to handle calls after reading samples from the streams).
It was also a bit unclear what a discontinuity implies and the new
Javadoc calls out the main use case for discontinuties and the intended
meaning of returning a discontinuity.
#minor-release
PiperOrigin-RevId: 543989124
(cherry picked from commit 7530c45cb60f212d9d65c24f40a45fe666416ad5)
