DOC: add description of pidfile in master-worker mode

Previously, pidfile was only described for daemon mode. In the case of
master-worker mode, the handling of pidfile is different from daemon mode,
so the description has been added.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index ca28806..83fb85a 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -1199,9 +1199,10 @@
   value is reported in the output of "haproxy -vv". See also "nbproc".
 
 pidfile <pidfile>
-  Writes PIDs of all daemons into file <pidfile>. This option is equivalent to
-  the "-p" command line argument. The file must be accessible to the user
-  starting the process. See also "daemon".
+  Writes PIDs of all daemons into file <pidfile> when daemon mode or writes PID
+  of master process into file <pidfile> when master-worker mode. This option is
+  equivalent to the "-p" command line argument. The file must be accessible to
+  the user starting the process. See also "daemon" and "master-worker".
 
 pp2-never-send-local
   A bug in the PROXY protocol v2 implementation was present in HAProxy up to
diff --git a/doc/haproxy.1 b/doc/haproxy.1
index 7554f9d..4c2d786 100644
--- a/doc/haproxy.1
+++ b/doc/haproxy.1
@@ -105,7 +105,8 @@
 .TP
 \fB\-p <pidfile>\fP
 Ask the process to write down each of its children's pids to this file
-in daemon mode.
+in daemon mode or ask the process to write down its master's pid to
+this file in master-worker mode.
 
 .TP
 \fB\-dk\fP