select.2: Move mislocated text describing the self-pipe text from BUGS to NOTES

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/select.2

@@ -407,6 +407,19 @@ Portable programs may wish to check for
 and loop, just as with
 .BR EINTR .
 
+On systems that lack
+.BR pselect (),
+reliable (and more portable) signal trapping can be achieved
+using the self-pipe trick.
+In this technique,
+a signal handler writes a byte to a pipe whose other end
+is monitored by
+.BR select ()
+in the main program.
+(To avoid possibly blocking when writing to a pipe that may be full
+or reading from a pipe that may be empty,
+nonblocking I/O is used when reading from and writing to the pipe.)
+
 Concerning the types involved, the classical situation is that
 the two fields of a
 .I timeval
@@ -577,19 +590,6 @@ Modern versions of glibc use the (race-free)
 .BR pselect ()
 system call on kernels where it is provided.
 
-On systems that lack
-.BR pselect (),
-reliable (and more portable) signal trapping can be achieved
-using the self-pipe trick.
-In this technique,
-a signal handler writes a byte to a pipe whose other end
-is monitored by
-.BR select ()
-in the main program.
-(To avoid possibly blocking when writing to a pipe that may be full
-or reading from a pipe that may be empty,
-nonblocking I/O is used when reading from and writing to the pipe.)
-
 Under Linux,
 .BR select ()
 may report a socket file descriptor as "ready for reading", while