spec: Remove requirements of Track Header Box.
spec: Move mention of file type branding.
spec: Remove mentions of sample_is_non_sync_sample within boxes related to track fragment.
spec: Move mention of presence of pre-roll indicator.

Also remove redundant descriptions.
spec: Move recommendations of timescale into 4.6 Trimming of Actual Duration.
spec: Remove 4.8.5 Sample Table Box.
spec: Fix an example of ChannelMapping.
spec: Remove 4.8.1 Edit Box.