kbfscrypto: fix merkle leaf comments and return values
kbfscrypto: fix merkle leaf comments and return values

Suggested by jzila.

Issue: #1589

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