src/bin/sage{,-runtests}: mention CPU time in --warn-long

The --warn-long flag now takes CPU seconds (not wall seconds) as its
argument. Here we update some command-line output to be explicit about
this fact.

Issue: sagemathGH-32981

src/bin/sage

@@ -428,7 +428,7 @@ usage_advanced() {
         echo "     --initial        -- only show the first failure per block"
         echo "     --debug          -- drop into PDB after an unexpected error"
         echo "     --failed         -- only test files that failed last test"
-        echo "     --warn-long [timeout] -- warning if doctest is slow"
+        echo "     --warn-long [timeout] -- warn if tests take too much CPU time"
         echo "     --only-errors    -- only output failures, not successes"
         echo "     --gc=GC          -- control garbarge collection (ALWAYS:"
         echo "                         collect garbage before every test; NEVER:"

src/sage/doctest/__main__.py

@@ -64,7 +64,7 @@ def _make_parser():
                         help="run as many doctests as possible in about 300 seconds (or the number of seconds given as an optional argument)")
     parser.add_argument("--warn-long", dest="warn_long", nargs='?',
                         type=float, default=-1.0, const=1.0, metavar="SECONDS",
-                        help="warn if tests take more time than SECONDS")
+                        help="warn if tests take more CPU time than SECONDS")
     # By default, include all tests marked 'sagemath_doc_html' -- see
     # https://github.com/sagemath/sage/issues/25345 and
     # https://github.com/sagemath/sage/issues/26110: