t0000: reword comments for "local" test

Commit 01d3a52 (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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

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 <[email protected]>, 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 <[email protected]>, 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 &&