gh-98930: improve the docstring of signal.strsignal (#99290)

Improves the docstring on signal.strsignal to make it explain when it returns a message, None, or when it raises ValueError.

Closes #98930

Co-authored-by: Gregory P. Smith <greg@krypto.org>

Doc/library/signal.rst

@@ -362,9 +362,9 @@ The :mod:`signal` module defines the following functions:
 
 .. 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.
+   Returns the description of signal *signalnum*, such as "Interrupt"
+   for :const:`SIGINT`. Returns :const:`None` if *signalnum* has no
+   description. Raises :exc:`ValueError` if *signalnum* is invalid.
 
    .. versionadded:: 3.8
 

Modules/signalmodule.c

@@ -626,13 +626,14 @@ signal.strsignal
 
 Return the system description of the given signal.
 
-The return values can be such as "Interrupt", "Segmentation fault", etc.
-Returns None if the signal is not recognized.
+Returns the description of signal *signalnum*, such as "Interrupt"
+for :const:`SIGINT`. Returns :const:`None` if *signalnum* has no
+description. Raises :exc:`ValueError` if *signalnum* is invalid.
 [clinic start generated code]*/
 
 static PyObject *
 signal_strsignal_impl(PyObject *module, int signalnum)
-/*[clinic end generated code: output=44e12e1e3b666261 input=b77914b03f856c74]*/
+/*[clinic end generated code: output=44e12e1e3b666261 input=238b335847778bc0]*/
 {
     const char *res;