hardlink: (usage) improve the descriptions of three options

hardlink: (usage) improve the descriptions of three options

The first description was too wordy for a --help text, the second
was vague and seemed to imply that it should be combined with the
preceding option, and the third failed to mention that it doesn't
actually link anything.

(Note: the choice for -m for --maximize and -M for --minimize is
unfortunate, as most people would guess the opposite, especially
since -s is used for --minimum-size and -S for --maximum-size.)

Signed-off-by: Benno Schulenberg <bensberg@telfort.nl>