doc: document the return value of {process,worker}.send()

Add documentation for the boolean value returned by process.send()
and worker.send().

Fixes: nodejs#26995

Author: shree-y

doc/api/cluster.md

@@ -459,6 +459,11 @@ if (cluster.isMaster) {
 }
 ```
 
+`worker.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.
+
 ## Event: 'disconnect'
 <!-- YAML
 added: v0.7.9

doc/api/process.md

@@ -1840,6 +1840,11 @@ 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.
 
+`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