Remove a field I accidentally added to struct bolt.

It was an intermediate stage in the refactor, unused in the final
result.
Add back documentation for portal vaults (dpeg).

I'm afraid that the code may not be sufficiently well explained, but I've
tried (and thankfully adding a new feature takes FAR less changes to the
code than it used to).

This partially reverts commit 6dca90ae4e3962b67a627fa450ad0d732f9cedf5.