Update diagram and primer to reflect issuer check addition

[elf-pavlik]

@uvdsl this was raised in #243 (review) , would you like to self-assign?
Otherwise I could self-assign and make PR next week.

[uvdsl]

If you can allocate the time next week for sure (I might, but not certain), then please go ahead - happy to review! Otherwise, I'll try and see if I can squeeze it in...