Stub out some docs
Die with proper error messages in the examples

Also just use definedness for the byte retvals.
Add ABSTRACT and SYNOPSIS
Add some more docs
Docs for nn_bind
Docs for nn_connect
Docs for nn_shutdown
Docs for nn_send
Add docs for nn_recv
Add docs for nn_allocmsg
Add docs for nn_errno
Allow loopback nn_devices using undef
Add docs for nn_device
Add docs for nn_term
Add docs for nn_sendmsg
Tweak the nn_send docs and markup
Wrap a long line
Add docs for nn_recvmsg
Document the protocols
Document the transports