Updates to documentation on metdata preservation after recent p4c cha… (#1057)
Updates to documentation on metdata preservation after recent p4c cha… (#1057)

* Updates to documentation on metdata preservation after recent p4c changes
In particular after this pull request was merged into p4c:
https://github.com/p4lang/p4c/pull/2902

* Remove some unnecessary changes from first commit
Should make this easier to review.

* Fix minor typo

* Update the v1model packet_length documentation based on code changes
done on 2021-Dec-16.
p4lang/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model