Fix fdo#80438: help entries of Excel new functions

write help entrie for FLOOR.PRECISE, CEILING.PRECISE and ISO.CEILING spreadsheet functions

Change-Id: I922fefbc84a7c7842a4510242e1dd477475810a5
Reviewed-on: https://gerrit.libreoffice.org/9948
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>