t0000: reword comments for "local" test

Commit 01d3a526ad (t0000: check whether the shell supports the "local"
keyword, 2017-10-26) added a test to gather data on whether people run
the test suite with shells that don't support "local".

After almost two years, nobody has complained, and several other uses
have cropped up in test-lib-functions.sh. Let's declare it acceptable to
use.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0000-basic.sh

@@ -25,16 +25,14 @@ try_local_x () {
 	echo "$x"
 }
 
-# This test is an experiment to check whether any Git users are using
-# Shells that don't support the "local" keyword. "local" is not
+# Check whether the shell supports the "local" keyword. "local" is not
 # POSIX-standard, but it is very widely supported by POSIX-compliant
-# shells, and if it doesn't cause problems for people, we would like
-# to be able to use it in Git code.
+# shells, and we rely on it within Git's test framework.
 #
-# For now, this is the only test that requires "local". If your shell
-# fails this test, you can ignore the failure, but please report the
-# problem to the Git mailing list <git@vger.kernel.org>, as it might
-# convince us to continue avoiding the use of "local".
+# If your shell fails this test, the results of other tests may be
+# unreliable. You may wish to report the problem to the Git mailing
+# list <git@vger.kernel.org>, as it could cause us to reconsider
+# relying on "local".
 test_expect_success 'verify that the running shell supports "local"' '
 	x="notlocal" &&
 	echo "local" >expected1 &&