DOC: Add --sigalert=NUM to swipl --help.

src/pl-init.c

@@ -1356,13 +1356,14 @@ usage(void)
     "    -l file                  Script source file\n",
     "    -s file                  Script source file\n",
     "    -p alias=path            Define file search path 'alias'\n",
-    "    -D name=value		  Set a Prolog flag\n",
+    "    -D name=value            Set a Prolog flag\n",
     "    -O                       Optimised compilation\n",
     "    --on-error=style         One of print, halt or status\n",
     "    --on-warning=style       One of print, halt or status\n",
     "    --tty[=bool]             (Dis)allow tty control\n",
     "    --packs[=bool]           Do (not) attach add-ons\n",
     "    --signals[=bool]         Do (not) modify signal handling\n",
+    "    --sigalert[=num]         Use signal num for alerting threads\n",
     "    --threads[=bool]         Do (not) allow for threads\n",
     "    --debug[=bool]           Do (not) generate debug info\n",
     "    --debug-on-interrupt[=bool] Trap the debugger on interrupt\n",