Clarified zmq_bind on icp://

doc/zmq_ipc.txt

@@ -39,9 +39,14 @@ When the address is `*`, _zmq_bind()_ shall generate a unique temporary
 pathname. The caller should retrieve this pathname using the ZMQ_LAST_ENDPOINT
 socket option. See linkzmq:zmq_getsockopt[3] for details.
 
-NOTE: any existing binding to the same endpoint shall be overridden. In this
-behavior, the 'ipc' transport is not consistent with the 'tcp' or 'inproc'
-transports.
+NOTE: any existing binding to the same endpoint shall be overridden. That is,
+if a second process binds to an endpoint already bound by a process, this
+will succeed and the first process will lose its binding. In this behavior,
+the 'ipc' transport is not consistent with the 'tcp' or 'inproc' transports.
+
+NOTE: the endpoint pathname must be writable by the process. When the endpoint
+starts with '/', e.g., `ipc:///pathname`, this will be an _absolute_ pathname.
+If the endpoint specifies a directory that does not exist, the bind shall fail.
 
 Connecting a socket
 ~~~~~~~~~~~~~~~~~~~
@@ -83,3 +88,7 @@ AUTHORS
 -------
 This 0MQ manual page was written by Pieter Hintjens <ph@imatix.com>,
 Martin Sustrik <sustrik@250bpm.com> and Martin Lucina <mato@kotelna.sk>.
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