[Bug 40852] select(2): document action of close()'ing a fd during a select()

@@ -411,6 +411,22 @@ when
 .B _GNU_SOURCE
 is defined.
 Since glibc 2.2.2 the requirements are as shown in the SYNOPSIS.
+.SS Multithreaded applications
+If a file descriptor being monitored by
+.BR select ()
+is closed in another thread, the result is unspecified.
+On some UNIX systems,
+.BR select ()
+unblocks and returns, with an indication that the file descriptor is ready
+(a subsequent I/O operation will likely fail with an error,
+unless another the file descriptor reopened between the time
+.BR select ()
+returned and the I/O operations was performed).
+On Linux (and some other systems),
+closing the file descriptor in another thread has no effect on
+.BR select ().
+In summary, any application that relies on a particular behavior
+in this scenario must be considered buggy.
 .SS "Linux Notes"
 The
 .BR pselect ()