DOC: configuration: more details about the master-worker mode

Add more details about the master-worker mode in the "master-worker"
global keyword.

Should fix issue haproxy#2198.

doc/configuration.txt

@@ -2140,10 +2140,18 @@ lua-prepend-path <string> [<type>]
 
 master-worker [no-exit-on-failure]
   Master-worker mode. It is equivalent to the command line "-W" argument.
-  This mode will launch a "master" which will monitor the "workers". Using
-  this mode, you can reload HAProxy directly by sending a SIGUSR2 signal to
-  the master. The master-worker mode is compatible either with the foreground
-  or daemon mode.
+
+  This mode will launch a "master" which will fork a "worker" after reading the
+  configuration to process the traffic. The master is used as a process manager
+  which will monitor the "workers".
+
+  Using this mode, you can reload HAProxy directly by sending a SIGUSR2 signal
+  to the master. Reloading will ask the master to read the configuration again
+  and fork a new worker. The previous worker will be kept until the end of its
+  jobs.
+
+  The master-worker mode is compatible either with the foreground or daemon
+  mode.
 
   By default, if a worker exits with a bad return code, in the case of a
   segfault for example, all workers will be killed, and the master will leave.