units: adjust Descriptions of various units

In general, it's not very usuful to repeat the unit name as the description.
Especially when the word is a common name and if somebody doesn't understand
the meaning immediately, they are not going to gain anything from the
repeat either, e.g. "halt", "swap".

In the status-unit-format=combined output parentheses are used around
Description, so avoid using parenthesis in the Description itself.

(cherry picked from commit 4fd3fc66)