fdo#86132 add useful tooltips to number formatting toolbar buttons

Change-Id: Iffd27c92cdec48b269c85ff9aef0483e742e36c7
Reviewed-on: https://gerrit.libreoffice.org/12416
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>