psignal(3): document return values of sig2str() and str2sig

Reviewed by:		bcr
Differential Revision:	https://reviews.freebsd.org/D52698

lib/libc/gen/psignal.3

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd May 10, 2025
+.Dd September 23, 2025
 .Dt PSIGNAL 3
 .Os
 .Sh NAME
@@ -141,6 +141,13 @@ The name in
 can be either the name of the signal, with or without the
 .Dq SIG
 prefix, or a decimal number.
+.Sh RETURN VALUES
+The
+.Fn sig2str
+and
+.Fn str2sig
+return 0 on success and -1 on translation failure.
+In the latter case the memory to store the translation result is left intact.
 .Sh SEE ALSO
 .Xr sigaction 2 ,
 .Xr perror 3 ,