t/README: clarify the test_have_prereq documentation

Clarify the test_have_prereq documentation so that it's clear in the
reader's mind when the text says "most common use of this directly"
what the answer to "as opposed to what?" is.

Usually this function isn't used in lieu of using the prerequisite
support built into test_expect_*, mention that explicitly.

This changes documentation that I added in commit
9a897893a7 ("t/README: Document the prereq functions, and 3-arg
test_*", 2010-07-02).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -611,9 +611,11 @@ library for your script to use.
 
  - test_have_prereq <prereq>
 
-   Check if we have a prerequisite previously set with
-   test_set_prereq. The most common use of this directly is to skip
-   all the tests if we don't have some essential prerequisite:
+   Check if we have a prerequisite previously set with test_set_prereq.
+   The most common way to use this explicitly (as opposed to the
+   implicit use when an argument is passed to test_expect_*) is to skip
+   all the tests at the start of the test script if we don't have some
+   essential prerequisite:
 
 	if ! test_have_prereq PERL
 	then