Add developer docs for unattended-upgrades

[eloquence]

Assuming we move forward with shipping unattended-upgrades with 1.8.0, we should add some developer documentation similar to what we have done with other components (e.g., securedrop-admin), given the complexity of interactions between various flags, timers, and configuration options.

This documentation ideally should well-specify the expected behavior -- when are package index updates expected to happen, when are package downloads expected to happen, when are reboots expected to happen, what happens if package updates trigger interactive prompts, etc. -- and includes important notes for testing/QA.

[eloquence]

(Note: I have no strong opinion on putting this documentation on the wiki vs. RTD - if anything I personally favor the wiki for developer docs - but I would suggest that we discuss that a bit more as we pick up issues like this one.)

[zenmonkeykstop]

I think there's a case for having this in the admin docs, if only to make troubleshooting more effective (as mentioned in ticket, by saying how the system should behave).