Revise the INTRO1_POW_EXT section

There were two different versions of this description in the doc, and they were both wrong in different ways. One was newer and implied the extension was encoded outside the encrypted section (it's inside). The other was older and had some outdated references and recommendations from when this was still a proposal, but it was in the correct location.

I consolidated the two and made the correct location of the extension explicit.

requested review from @ahf

approved this merge request

Looks good!

assigned to @beth

merged

mentioned in commit 81942fbb

mentioned in merge request !254 (merged)