t/README: unify documentation of test function args

Document all test function arguments in the same way.

While at it, tweak the description of test_path_is_* (thanks to Junio),
and correct some grammatical errors.
Signed-off-by: NMathias Lafeldt <misfire@debugon.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/README

@@ -379,7 +379,7 @@ library for your script to use.
 
  - test_expect_success [<prereq>] <message> <script>
 
-   Usually takes two strings as parameter, and evaluates the
+   Usually takes two strings as parameters, and evaluates the
    <script>.  If it yields success, test is considered
    successful.  <message> should state what it is testing.
 
@@ -390,7 +390,7 @@ library for your script to use.
 	    'tree=$(git-write-tree)'
 
    If you supply three parameters the first will be taken to be a
-   prerequisite, see the test_set_prereq and test_have_prereq
+   prerequisite; see the test_set_prereq and test_have_prereq
    documentation below:
 
 	test_expect_success TTY 'git --paginate rev-list uses a pager' \
@@ -446,7 +446,7 @@ library for your script to use.
    Merges the given rev using the given message.  Like test_commit,
    creates a tag and calls test_tick before committing.
 
- - test_set_prereq SOME_PREREQ
+ - test_set_prereq <prereq>
 
    Set a test prerequisite to be used later with test_have_prereq. The
    test-lib will set some prerequisites for you, see the
@@ -456,7 +456,7 @@ library for your script to use.
    test_have_prereq directly, or the three argument invocation of
    test_expect_success and test_expect_failure.
 
- - test_have_prereq SOME PREREQ
+ - 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
@@ -526,12 +526,13 @@ library for your script to use.
 
    Check whether a file has the length it is expected to.
 
- - test_path_is_file <file> [<diagnosis>]
-   test_path_is_dir <dir> [<diagnosis>]
+ - test_path_is_file <path> [<diagnosis>]
+   test_path_is_dir <path> [<diagnosis>]
    test_path_is_missing <path> [<diagnosis>]
 
-   Check whether a file/directory exists or doesn't. <diagnosis> will
-   be displayed if the test fails.
+   Check if the named path is a file, if the named path is a
+   directory, or if the named path does not exist, respectively,
+   and fail otherwise, showing the <diagnosis> text.
 
  - test_when_finished <script>