pwrite(2): add a BUGS section

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

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

(cherry picked from commit 504e64af32ba6c62fdcc894a3b1da76061c64796)