[llvm-ar][nfc] Reword help message to be less ambiguous on what p and t do.

The current help msg isn't super clear on whether t prints the content of the files or just the list of files.
(I'd certainly thought it'd print the list of files, and accidentally had a bunch of "gargabe" printed to my terminal).
Similarly, t sounded like it'd do what p actually did.

Differential Revision: https://reviews.llvm.org/D109018