round to multiple of absolute significance, fdo#71720 follow-up

The new functions FLOOR.PRECISE, CEILING.PRECISE and ISO.CEILING always
round to a multiple of the absolute value of the significance given,
returning the mathematical floor/ceiling in all cases.

Also changed the test doc to use some meaningful values for these
functions.

Change-Id: Id5a26092838765143e2d308afa49e7119107dac5