h264: update AVC spec reference to be explicit and consistent

Brad Hards requested to merge bradh/gstreamer:h264_apidoc_2021-11-07 into main

There are a couple of references that are newer, and are against the 2020 version, but some that are older and don't point to the right section in ISO/IEC 14496-10:2020(E).

This adds references where appropriate, and updates to reflect changes.

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