DOC: explicitly mention the limits of abstract namespace sockets

DOC: explicitly mention the limits of abstract namespace sockets

Listening to an abstract namespace socket is quite convenient but
comes with some drawbacks that must be clearly understood when the
socket is being listened to by multiple processes. The trouble is
that the socket cannot be rebound if a new process attempts a soft
restart and fails, so only one of the initially bound processes
will still be bound to it, the other ones will fail to rebind. For
most situations it's not an issue but it needs to be indicated.