diff --git a/doc/api/cluster.md b/doc/api/cluster.md
index 340c2968cb6fb0..4fe41346b797f7 100644
--- a/doc/api/cluster.md
+++ b/doc/api/cluster.md
@@ -459,6 +459,21 @@ if (cluster.isMaster) {
 }
 ```
 
+The optional `callback` is a function that is invoked after the message is
+sent but before the worker or master may have received it.
+The function is called with a single argument: `null` on success,
+or an [`Error`][] object on failure.
+
+If no `callback` function is provided and the message cannot be sent, an
+`'error'` event will be emitted by the [`Worker`][] object. This can
+happen, for instance, when the worker has already exited.
+
+
+`worker.send()` will throw an error if the channel has closed or when the
+backlog of unsent messages exceeds a threshold that makes it unwise to send
+more. Otherwise, the method returns `true`. The `callback` function can be
+used to implement flow control.
+
 ## Event: 'disconnect'
 <!-- YAML
 added: v0.7.9
@@ -847,8 +862,10 @@ socket.on('data', (id) => {
 [`child_process` event: `'message'`]: child_process.html#child_process_event_message
 [`cluster.settings`]: #cluster_cluster_settings
 [`disconnect`]: child_process.html#child_process_subprocess_disconnect
+[`Error`]: errors.html#errors_class_error
 [`kill`]: process.html#process_process_kill_pid_signal
 [`process` event: `'message'`]: process.html#process_event_message
 [`server.close()`]: net.html#net_event_close
+[`Worker`]: #cluster_class_worker
 [`worker.exitedAfterDisconnect`]: #cluster_worker_exitedafterdisconnect
 [Child Process module]: child_process.html#child_process_child_process_fork_modulepath_args_options
diff --git a/doc/api/process.md b/doc/api/process.md
index c868f01fa1d3e8..7437f000f30320 100644
--- a/doc/api/process.md
+++ b/doc/api/process.md
@@ -1840,6 +1840,21 @@ If Node.js was not spawned with an IPC channel, `process.send()` will be
 The message goes through serialization and parsing. The resulting message might
 not be the same as what is originally sent.
 
+The optional `callback` is a function that is invoked after the message is sent
+but before the parent process may have received it.
+The function is called with a single argument: `null` on success,
+or an [`Error`][] object on failure.
+
+If no `callback` function is provided and the message cannot be sent, an
+`'error'` event will be emitted by the [`Process`][] object. This can
+happen, for instance, when the process has already exited.
+
+
+`process.send()` will return false if the channel has closed or when the
+backlog of unsent messages exceeds a threshold that makes it unwise to send
+more. Otherwise, the method returns `true`. The `callback` function can be
+used to implement flow control.
+
 ## process.setegid(id)
 <!-- YAML
 added: v2.0.0
@@ -2335,3 +2350,4 @@ cases:
 [process_emit_warning]: #process_process_emitwarning_warning_type_code_ctor
 [process_warning]: #process_event_warning
 [report documentation]: report.html
+[`Process`]: #process_process