swapon: rename the new --annotation option to --annotate

swapon: rename the new --annotation option to --annotate

The form --annotation gave the impression that after the subsequent
"=" comes the text that each column name should be annotated with.
Furthermore, the other two options with optional arguments (--show
and --discard) are in the imperative, so follow that style and use
--annotate instead.

(In the bargain, this allows aligning the descriptions in the help
text perfectly again.)

Also, improve the description of --annotate in the man page somewhat.

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