More doc about garbage collection

Created by: muffato

(This is a follow up from a discussion on Slack https://genomes-ebi.slack.com/archives/C0F9K0G2E/p1543587614000800 )

Here I add more documentation about the garbage collection process, with a couple of examples and a note about UNKNOWN errors.

PS: I will change the base branch once the other pull-request is merged