signal: add strsignal() (#6017)

Co-authored-by: Vajrasky Kok <sky.kok@speaklikeaking.com>
diff --git a/Doc/library/signal.rst b/Doc/library/signal.rst
index 67eaa2c..3f17e08 100644
--- a/Doc/library/signal.rst
+++ b/Doc/library/signal.rst
@@ -207,6 +207,15 @@
    installed from Python.
 
 
+.. function:: strsignal(signalnum)
+
+   Return the system description of the signal *signalnum*, such as
+   "Interrupt", "Segmentation fault", etc. Returns :const:`None` if the signal
+   is not recognized.
+
+   .. versionadded:: 3.8
+
+
 .. function:: pause()
 
    Cause the process to sleep until a signal is received; the appropriate handler