tune2fs: edit dire warning about check intervals

tune2fs: edit dire warning about check intervals

Time & mount-count based checks have been off by default for quite some
time now, but the dire warning about disabling them remains in the
tune2fs manpage, which is confusing.  We did "strongly consider
the consequences" and disabled it by default, no need to scare the
user about it now.  Inform the user of the consequences in a more
measured tone.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>