Bug 448858 – In <test_one.sh>, document new "check-one" call mode too

(Av1) Add command example

tools/test-harness/xpcshell-simple/test_one.sh

@@ -49,7 +49,9 @@ XPCOM_DEBUG_BREAK=stack-and-abort; export XPCOM_DEBUG_BREAK
 
 exit_status=0
 
-# Launch single tests by calling:
+# Launch a single test by calling:
+# make SOLO_FILE=(filename) -C (test directory) check-one
+# or
 # make SOLO_FILE=(filename) -C (test directory) check-interactive
 # js>_execute_test();
 # js>quit();