t0008: document test_expect_success_multi

test_expect_success_multi() helper function warrants some explanation,
since at first sight it may seem like generic test framework plumbing,
but is in fact specific to testing check-ignore, and allows more
thorough testing of the various output formats without significantly
increase the size of t0008.

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0008-ignores.sh

@@ -75,6 +75,16 @@ test_check_ignore () {
 	stderr_empty_on_success "$expect_code"
 }
 
+# Runs the same code with 3 different levels of output verbosity,
+# expecting success each time.  Takes advantage of the fact that
+# check-ignore --verbose output is the same as normal output except
+# for the extra first column.
+#
+# Arguments:
+#   - (optional) prereqs for this test, e.g. 'SYMLINKS'
+#   - test name
+#   - output to expect from -v / --verbose mode
+#   - code to run (should invoke test_check_ignore)
 test_expect_success_multi () {
 	prereq=
 	if test $# -eq 4