setsid: document fork(2) usage

Addresses: https://github.com/karelzak/util-linux/issues/518
Signed-off-by: Karel Zak <kzak@redhat.com>

Documentation/TODO

@@ -45,6 +45,11 @@ cleanup lib/path.c:
      to /sys snapshots (e.g. lsblk regression tests)
 	
 
+setsid
+------
+  - add --fork to always call fork(2) before setsid(2)
+    https://github.com/karelzak/util-linux/issues/518
+
 lscpu
 -----
   - read cpuid and uname information from file if --sysroot is specified, then

sys-utils/setsid.1

@@ -10,7 +10,10 @@ setsid \- run a program in a new session
 .RI [ arguments ]
 .SH DESCRIPTION
 .B setsid
-runs a program in a new session.
+runs a program in a new session. The command calls
+.BR fork (2)
+if already a process group leader. Otherwise, it executes a program in the
+current process.
 .SH OPTIONS
 .TP
 .BR \-c , " \-\-ctty"