chore: evaluate closed PR about GTP Extension Header Feature Documentation

[lucaaamaral]

Problem

The PR magma/magma#12883 proposed documentation about GTP Extension Header Feature Documentation that was approved but never merged because of CI checks and tight rules.

Solution

Evaluate if the content provided is still relevant and applicable, update if necessary.
Report the findings and open a new PR if its content is still relevant.

Non-goals

[lucaaamaral]

Micky's comments: we are good to merge it.