For tcsetpgrp(3), mention SIGTTOU can be raised.
For tcgetpgrp(3), mention SIGTTIN can be raised.
In both cases, suggest options to handle the signal.
Add cross references to sigaction and signal.
From the original PR:
I recently had some difficulties when trying to use tcsetpgrp(3)
from a process trying to bring itself as the foreground process
group leader of its controlling terminal.
The process got a SIGTTOU sent, that was unexpected as this signal
is not mentioned at all in the tcsetpgrp(3) manpage.
An explanation of the role of SIGTTOU and its usage relative to
tcsetpgrp(3) should be added to the manual page as it plays an
important role in the correct application of this library function.
PR:264971