pwrite(2): add a BUGS section

Add a BUGS section about using pwrite(2) when O_APPEND is set on the fd.

MFC after:	3 days
Submitted by:	Ka Ho Ng <khng300@gmail.com>
Reviewed by:	gbe, yuripv
Differential Revision:	https://reviews.freebsd.org/D28372

lib/libc/sys/write.2

@@ -28,7 +28,7 @@
 .\"     @(#)write.2	8.5 (Berkeley) 4/2/94
 .\" $FreeBSD$
 .\"
-.Dd October 25, 2020
+.Dd February 11, 2021
 .Dt WRITE 2
 .Os
 .Sh NAME
@@ -298,3 +298,17 @@ The
 .Fn write
 function appeared in
 .At v1 .
+.Sh BUGS
+The
+.Fn pwrite
+system call appends the file without changing the file offset if
+.Dv O_APPEND
+is set, contrary to
+.St -p1003.1-2008
+where
+.Fn pwrite
+writes into
+.Fa offset
+regardless of whether
+.Dv O_APPEND
+is set.