getsid.2: Rework description to be somewhat clearer

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/getsid.2

@@ -1,4 +1,5 @@
 .\" Copyright (C) 1996 Andries Brouwer (aeb@cwi.nl)
+.\" and Copyright (C) 2016 Michael Kerrisk <mtk.manpages@gmail.com>
 .\"
 .\" %%%LICENSE_START(GPLv2+_DOC_FULL)
 .\" This is free documentation; you can redistribute it and/or
@@ -50,11 +51,14 @@ _XOPEN_SOURCE\ >=\ 500
 .SH DESCRIPTION
 .I getsid(0)
 returns the session ID of the calling process.
-.I getsid(p)
+.BR getsid ()
 returns the session ID of the process with process ID
-.IR p .
-(The session ID of a process is the process ID of the
-session leader.)
+.IR pid .
+If
+.I pid
+is 0,
+.BR getsid ()
+returns the session ID of the calling process.
 .SH RETURN VALUE
 On success, a session ID is returned.
 On error, \fI(pid_t)\ \-1\fP will be returned, and
@@ -81,6 +85,10 @@ POSIX.1-2001, POSIX.1-2008, SVr4.
 .SH NOTES
 Linux does not return
 .BR EPERM .
+
+See
+.BR credentials (7)
+for a description of sessions and session IDs.
 .SH SEE ALSO
 .BR getpgid (2),
 .BR setsid (2),