Update pgm-daemon docs with the headless option.

This was forgotten in eed56714e0e824b282e38d18149a0242fc13da30.

Change-Id: Ic5883c8a44f1c4c1312f3cd7122b10aedf8a8b03
Signed-off-by: Sasa Zivkov <sasa.zivkov@sap.com>
diff --git a/Documentation/pgm-daemon.txt b/Documentation/pgm-daemon.txt
index 1c1d343..3ffcb40 100644
--- a/Documentation/pgm-daemon.txt
+++ b/Documentation/pgm-daemon.txt
@@ -14,6 +14,7 @@
 	[\--enable-sshd | \--disable-sshd]
 	[\--console-log]
 	[\--slave]
+	[\--headless]
 
 DESCRIPTION
 -----------
@@ -58,6 +59,10 @@
 	Send log messages to the console, instead of to the standard
 	log file '$site_path/logs/error_log'.
 
+\--headless::
+	Don't start the default Gerrit UI. May be useful when Gerrit is
+	run with an alternative UI.
+
 CONTEXT
 -------
 This command can only be run on a server which has direct