doc: Disambiguate modification time vs timestamp

[vp-elitnet]

Description of --changed-within has the following statement.

Files with modification times greater than the argument will be returned.

At least for me the natural interpretation of "modification time" is relative, e.g. 2 weeks ago, 24 hours ago, etc. Using this interpretation the statement above is opposite of the actual behaviour: modification time greater than (i.e. more than) 24 hour would match files older than the argument, not newer.

I think the statement intended to say "modification timestamps greater than", which would make it unambiguous, i.e. prevent the incorrect interpretation. Fixed descriptions of both --changed-within and --changed-before accordingly.