Update to reflect the fact that pipe() is a wrapper around the pipe2()

system call.

Reviewed by:	jhb, wblock
MFC after:	5 days
Sponsored by:	DAPRA, AFRL
Differential Revision:	https://reviews.freebsd.org/D6948

lib/libc/sys/pipe.2

@@ -28,7 +28,7 @@
 .\"     @(#)pipe.2	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd June 22, 2016
+.Dd July 20, 2016
 .Dt PIPE 2
 .Os
 .Sh NAME
@@ -109,6 +109,18 @@ The bidirectional nature of this implementation of pipes is not
 portable to older systems, so it is recommended to use the convention
 for using the endpoints in the traditional manner when using a
 pipe in one direction.
+.Sh IMPLEMENTATION NOTES
+The
+.Fn pipe
+function calls the
+.Fn pipe2
+system call.
+As a result, system call traces such as those captured by
+.Xr dtrace 1
+or
+.Xr ktrace 1
+will show calls to
+.Fn pipe2 .
 .Sh RETURN VALUES
 .Rv -std pipe
 .Sh ERRORS
@@ -157,3 +169,10 @@ The
 .Fn pipe2
 function appeared in
 .Fx 10.0 .
+.Pp
+The
+.Fn pipe
+function became a wrapper around
+.Fn pipe2
+in
+.Fx 11.0 .