doc: document and test that methods return this

Also, add tests to ensure they will always return this, and to confirm
they return this when these doc changes are back-ported to earlier
release lines.

PR-URL: #13553
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-net-end-close.js

@@ -14,7 +14,7 @@ const s = new net.Socket({
   },
   writable: false
 });
-s.resume();
+assert.strictEqual(s, s.resume());
 
 const events = [];
 

test/parallel/test-net-server-close.js

@@ -23,7 +23,7 @@ let server = net.createServer(function(c) {
   sockets.push(c);
 
   if (sockets.length === 2) {
-    server.close();
+    assert.strictEqual(server.close(), server);
     sockets.forEach(function(c) { c.destroy(); });
   }
 });
@@ -32,7 +32,7 @@ server.on('close', function() {
   events.push('server');
 });
 
-server.listen(0, function() {
+assert.strictEqual(server, server.listen(0, function() {
   net.createConnection(this.address().port);
   net.createConnection(this.address().port);
-});
+}));

test/parallel/test-net-socket-local-address.js

@@ -34,7 +34,10 @@ function connect() {
 
   conns++;
   client.once('close', connect);
-  client.connect(server.address().port, common.localhostIPv4, () => {
-    clientLocalPorts.push(client.localPort);
-  });
+  assert.strictEqual(
+    client,
+    client.connect(server.address().port, common.localhostIPv4, () => {
+      clientLocalPorts.push(client.localPort);
+    })
+  );
 }

test/parallel/test-net-stream.js

@@ -39,7 +39,7 @@ const server = net.createServer(function(socket) {
 }).listen(0, function() {
   const conn = net.connect(this.address().port);
   conn.on('data', function(buf) {
-    conn.pause();
+    assert.strictEqual(conn, conn.pause());
     setTimeout(function() {
       conn.destroy();
     }, 20);