Add some words clarifying that rename(2) does nothing when the 'from' and

'to' args are the same file.  Wording borrowed from POSIX.1-2017, but
the freebsd code to implement this behavior was added in 2002 (r103180).

lib/libc/sys/rename.2

@@ -28,7 +28,7 @@
 .\"     @(#)rename.2	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd September 15, 2017
+.Dd June 21, 2018
 .Dt RENAME 2
 .Os
 .Sh NAME
@@ -76,6 +76,15 @@ is a symbolic link,
 the symbolic link is renamed,
 not the file or directory to which it points.
 .Pp
+If
+.Fa from
+and
+.Fa to
+resolve to the same directory entry, or to different directory
+entries for the same existing file,
+.Fn rename
+returns success without taking any further action.
+.Pp
 The
 .Fn renameat
 system call is equivalent to