Improve documentation of --keep-redundant.

Improve documentation of --keep-redundant.

The help text for --keep-redundant was rather convoluted. This commit
shortens the text with no essential semantic change.

The patch also removes the sentence that encourages to specify this
option if the destination directory is only used for snapshots. After
all, most file systems allow to create an insane number of files,
so keeping snapshots around forever can result in a file system that
can no longer be checked or repaired due to the excessive number of
used inodes.