Document container functions
Document container functions

Summary:
Add some documents to the container functions, in particular documenting
the procedure we use to create a new container from scratch/from snapshot
since nobody previously seemed to know what actually happens in the full
stack.

Test Plan: Doc.

Reviewers: jukka

Reviewed By: jukka

Subscribers: mkedia, vishal

Differential Revision: https://tails.corp.dropbox.com/D117673

dropbox/changes-client,dropbox/changes-client