`net_kernel:allowed/0`, specs and docs

[MarkoMin]

Few changes:

1. net_kernel:allowed/0 is speced and documented. Notice that it is already listed in "documented API exports", but for some reason docs were missing. I think it's a must to have it documented, because if you don't then the user can't figure out if net_kernel:allow/1 was called in the past, therefore blocking nodes with the same cookie to connect (which I believe can be very frustrating and and to debug!). This is just a pure getter function, I see no threat exposing it.
2. net_kernel:allow/1 was missing ignored value it can return. You'll get it if e.g. your node is started without (s)name.
3. 2 small typos in comments