docs: Add note to upgrade guide about manifest.hls.useFullSegmentsForStartTime (#4213)
docs: Add note to upgrade guide about manifest.hls.useFullSegmentsForStartTime (#4213)

This was removed in the release, but we failed to note this in the
upgrade guide.

Closes #4210
shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player