opus

Various small improvements.

Remove a redundant phrase from the previous commit.
Thanks to Tim for pointing this out.

Clarify what 'skipped' means.

Give a reference for the vorbis granulepos-trimming.

Commas and semicolons in the resampling decision list.

Capital SHOULD on treating unknown channel mapping families
as discrete.

Clarify why goes in the vendor string. Maybe needs an example?
'gstoggmux 1.0.12; libopus 1.2.3'

Remove a redundant word. 'could' already covers 'potential' and
the line is stronger without it.