Minor copy edit in agedu's man page, and also added another paragraph

in the limitations section mentioning that it won't work with noatime.

(Of course that _should_ be obvious, but it's still worth mentioning
it as a thing to check someone else hasn't inflicted on you if agedu
mysteriously isn't doing anything useful, and it's also worth
commenting on relatime.)

git-svn-id: svn://svn.tartarus.org/sgt/agedu@9480 cda61777-01e9-0310-a592-d414129be87e