Correct the description in com/sun/star/animation/XIterateContainer.
Both sd and xmloff take the value of the interval as second when
invoking setIterateInterval() and getIterateInterval(), so does
the slideshow. Referring to the duration of the container is
only implmented in UI level and is not related to API.

Change-Id: Ieb86c5b2b99c7a58314241ed62f273ba424e9962
Reviewed-on: https://gerrit.libreoffice.org/59409
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>