Add more documentation about EntryGuardAddedBy in the state file
Document better how EntryGuardAddedBy date is calculated and written in the state file (well, more documentation about all the state file would be also good).
Should we also change the way the date is calculated?
The EntryGuardAddedBy dates in the state file are not sequential because the date is calculated as the real date minus a random date between the real and 30 days before .
The reason for this is to avoid revealing the real date the user was online at that date to that entry guard.
The only existent documentation is in doc/state-contents.txt
 e->chosen_on_date = time(NULL) - crypto_rand_int(36002430);