Martin's issue 11 - Expand rewrite pop/push descriptions

[rgwilton]

o ietf-flexible-encapsulation

The descriptions for pop/push are a bit terse. It seems to assume
that readers already know what this (from somewhere) is so it
doesn't need to be described. If this is intended, perhaps add a
reference to where this is described.

[rgwilton]

This isn't any standard reference for this functionality. I have tried to expand the descriptions in the YANG model, please can you check the latest version of the draft to see if you think these are sufficient.

[samans]

Reviewed, looks to be implemented.