Comment on the return value for pipe in the man

Notes:
See intro(2) DIAGNOSTICS.

Reference: /n/sources/patch/sorry/pipeman
Date: Fri Mar 23 15:00:06 CET 2007
Signed-off-by: paurea@gmail.com
Reviewed-by: geoff

--- /sys/man/2/pipe	Fri Mar 23 14:57:41 2007
+++ /sys/man/2/pipe	Fri Mar 23 14:57:38 2007
@@ -63,6 +63,7 @@
 .IR read (2),
 .IR pipe (3)
 .SH DIAGNOSTICS
+Returns a negative number if an error occurred.
 Sets
 .IR errstr .
 .SH BUGS