[3.13] pythongh-139843: Document signal.SIGQUIT to fix Sphinx references (pythonGH-139844) (python#139886)

pythongh-139843: Document `signal.SIGQUIT` to fix Sphinx references (pythonGH-139844)
(cherry picked from commit 1f87d52)

Co-authored-by: Weilin Du <108666168+LamentXU123@users.noreply.github.com>

Author: miss-islington

Doc/library/signal.rst

@@ -205,6 +205,12 @@ The variables defined in the :mod:`signal` module are:
 
    .. availability:: Unix.
 
+.. data:: SIGQUIT
+
+   Terminal quit signal.
+
+   .. availability:: Unix.
+
 .. data:: SIGSEGV
 
    Segmentation fault: invalid memory reference.