t/README: unify documentation of test function args
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: Mathias Lafeldt <040cb4e9e85a2ad4d7fdb95c42ca88daea898bc4@debugon.org>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git