favorites: clarify context for AddAsync in comment
favorites: clarify context for AddAsync in comment

Suggested by @akalin-keybase.

Issue: #969

keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client