Add comments to user.yml based on @bnvk's input

These are variables @bnvk asked about, which probably means they'd be
confusing to a normal user.
Let migration be disabled by rming mail_migration

> @PiPeep sounds good, definitely better than before! Might want to make
> mail_migration be "configurable" from here in this yml file, instead
> of having to go edit the .sh file as well
Merge pull request #45 from taoeffect/comment-user.yml

Add comments to user.yml based on @bnvk's input