readv2: Note preadv2(..., RWF_NOWAIT) bug in BUGS section

To save the next person before they fall foul of it.  See
<https://lore.kernel.org/linux-fsdevel/fea8b16d-5a69-40f9-b123-e84dcd6e8f2e@www.fastmail.com/T/#u>
and <tokio-rs/tokio#3803> for more information.

Signed-off-by: Will Manley <will@williammanley.net>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/readv.2

@@ -243,7 +243,9 @@ If some data was successfully read, it will return the number of bytes read.
 If no bytes were read, it will return \-1 and set
 .IR errno
 to
-.BR EAGAIN .
+.BR EAGAIN
+(but see
+.BR BUGS ")."
 Currently, this flag is meaningful only for
 .BR preadv2 ().
 .TP
@@ -425,6 +427,13 @@ iov[1].iov_len = strlen(str1);
 nwritten = writev(STDOUT_FILENO, iov, 2);
 .EE
 .in
+.SH BUGS
+Linux v5.9 and v5.10 have a bug where
+.BR preadv2()
+with the
+.BR RWF_NOWAIT
+flag may return 0 even when not at end of file.  See
+https://lore.kernel.org/linux-fsdevel/fea8b16d-5a69-40f9-b123-e84dcd6e8f2e@www.fastmail.com/T/#u
 .SH SEE ALSO
 .BR pread (2),
 .BR read (2),